Malfunctioning rear lights

The obvious suspects are the fuses, the bulbs and—especially if more than one light is failing—the ground. Removing the combination lights is easy: on Berline (Hatchback) models press the lens forward and simultaneously pinch the retaining clip firmly from within the luggage compartment, withdraw the unit, separate the lens and the bulb holder assembly by pressing the retaining catch; on Break (Estate) models raise the tailgate, remove the two lens securing screws, remove the screw retaining the assembly, then withdraw the unit.

If the corresponding left light works but the right one fails or vice versa, fuses are generally out of question (however, the tail lights feed through different fuses, LH tail light and number plate lights through F12 while RH tail light through F11). The current arrives behind the left rear light assembly, goes over to the right side, then comes back to be grounded with a bolt on the left side. Check the bulbs and clean the connectors in the light assemblies. If the problem still persists, undo the grounding bolt, clean the connections until shiny, spray them liberally with silicon spray and retighten.

Poor grounding often causes ghost lights: for instance, the stop lights flash together with the traffic indicators (often not at full brightness) or they go out when the indicator lights up, or any similar misbehavior. Just remove the left lamp cluster, undo, clean, spray with silicon and re-fasten the grounding bolt behind it. Clean all connectors of both lamp clusters.

If you didn't yet cure the problem, start to trace back the wiring. Although you have just cleaned the connectors and checked the bulbs (meaning that, if they don't work, the feed couldn't reach here), take the time to check the feeding connector of the left light assembly with a test lamp or voltmeter.

Series I
Series II
1 rear foglights
2 reversing lights
3 right turn indicator
4 left turn indicator
5 stop lights
6 tail lights right tail light
7 left tail light

If you are surprised to find 12 V at the indicated pin, recheck the connectors and bulbs in the rear (bulb holders tend to corrode over time and will not work even if a brand new and perfect bulb is sitting in them), going through them all with a test lamp. If there is no voltage at this connector, go on to the specific troubleshooting section.

We describe the various circuits in a troubleshooting order rather than going through them blindly from the feed to the lamp. Some of the connectors involved are hard to reach and may require removal of internal trim parts. So, we should try to avoid unnecessary work by ruling out possible failures using easy checking points.

You should check every connector we mention with a test lamp or voltmeter, disconnect, clean and reconnect them firmly as you go along (even if they still make good contact, a little bit of cleaning pays off by helping you avoid many new failures in the future). Check that you still have the positive feed on every connection. When we speak about checking a fuse, do not only verify that it is still good but check with the lamp the socket which the fuse is seated in.

Rear foglights (Series I, before 1986)

Check the rear foglight warning lamp. If it lights up when you switch the foglights on, check pin 10 of I/3 (10-pin white). If your test lamp lights up there, only the flat grey cable going back to the lamps can be at fault. Alas, this requires the removal of many interior trim panels and carpets.

If the warning lamp is not lit, check fuse 11, then pin 3 of IV/1 (10-pin yellow). If you cannot measure 12V here but the feed is present at pin 10 of the same connector, you have to check the switch. As a quick proof, jumper these two pins: both the waring light and the rear foglights should work now.

For access to the switch, prise free the central panel of the left steering column switch [yellow], remove the two retaining screws now exposed, withdraw the central switch unit [green] and the selector control unit [red]. Prise free the upper and lower switches [blue] as well. Detach the connectors from the main loom and withdraw the multi-connector panel.

Rear foglights (Series II, after 1986)

Check the rear foglight switch. If it lights up when you switch it on, go to A.

Check pin 3 of the main board connector IV/1 (10-pin yellow). If OK, check Fuse 10. If OK, check pin 6 of II/1 (10-pin brown). ???

Either remove the instrument panel or try to prise free the foglight switch from the facia panel. The second solution is possible but difficult and carries the risk of damaging the retaining clips or the facia panel. Once you removed the facia, you can push out the switch from the rear while compressing its retaining clips.

A. The current does reach Fuse 10. Check pin 10 of I/4 (10-pin white). If OK, the next stop is already the connector at the rear, hence, the flat grey cable is at fault. Bother, this requires the removal of many interior trim parts.

Reversing lights

Look down between the battery and the engine, a little bit to the front, where the clutch cable passes over the gearbox (bigger models like BX 19 and GTi have lots of gadgets in the way). You'll find a switch screwed into the gearbox housing with two wires connected. Remove the wires, clean the connectors. With the ignition on, look for 12V feed in one of the wires (BX 14 models from 1988 have a mauve sleeve on this wire).

If you find the feed, go to A; otherwise, check Fuse 1 and pin 4 of V/5 (4-pin yellow). If you have a feed there, look for pin 2 of a 8-pin white connector (or pin 5 of a 7-pin brown connector in BX 14s after 1988) between the fuse and the switch in the gearbox. Either the connector or the wiring inbetween is at fault.

If there is no feed, check the other circuits from this fuse for correct functioning. If none of them works, you have a problem with the ignition switch or its connectors.

A. Reconnect the wire you checked for 12V in to the switch in the gearbox (doesn't matter to which output pin). Put the gear into reverse and check for 12V on the other output pin of the switch. If none, the switch should be replaced. You can replace it without draining the transmission (except for 150A and 150C engines), however, undoing it might require the removal of its environment for access, depending on the model (for instance, the air intake hoses block the way on fuel injected models). You could also try to reach it from beneath.

If the switch was OK, reconnect the other wire, too. Check pin 2 of V/5 (4-pin yellow) and pin 9 of I/4 [I/3 on pre-1986 models] (10-pin white). If no feed, look for pin 3 of a 8-pin white connector (or pin 6 of a 7-pin brown connector in BX 14s after 1988) back to the switch in the gearbox. Either the connector or the wiring inbetween is at fault.

From I/4 [or I/3 on pre-1986 models] the wiring goes straight back to the rear lamp assembly. If you have feed at the mainboard and none at the other end, the flat grey cable is broken somewhere. Alas, this requires the removal of many interior trim panels and carpets.

Stop lights

Locate the stop light switch beneath the footbrake pedal. Remove the wires, clean the connectors. With the ignition on, look for 12V feed in the green cable (later cars have a mauve sleeve on the wire).

If there is no feed here, check Fuse 3 and pin 1 of III/2 (6-pin brown). If you have no feed, probably the complete dashboard lighting and the screen washers/wipers are not working either. In this case, check pin 2 of IV/4 (6-pin yellow) and, finally, the ignition switch and its connectors.

But if you found current in the cable, reconnect this one to the footbrake pedal switch, leave the other alone and check for feed at the other output pin of the switch while pressing the break pedal. Your test lamp should light, or you have to renew the switch.

If you had 12V, reconnect the other wire to the switch and go on to pin 5 of III/2 (6-pin brown) and later to pin 6 of I/4 [or I/3 on pre-1986 models] (10-pin white). If you lost the feed somewhere, you have found the culprit. If you still have it at the last connector, then the flat grey cable going back to the rear lamps from here is broken. Alas, this requires the removal of many interior trim panels and carpets.

Tail lights


Higher wattage bulbs

A common idea is to use overwattage (100/50 or even 100/80 W) bulbs. The main problem is the additional heat generated which can harm the reflector and the housing. If you decide to experiment with them, an additional relay is absolutely required as the switch on the dashboard is not expected to handle such high currents. Don't forget about thicker wires and appropriate fuses, either.

And even before that, you can consider what Julian MARSH wrote: "I am using Ring Xenon H1 bulbs in my XM; these are regular H1 lamps which employ xenon rather than halogen as the gas. According to Ring, they are 30% brighter than regular bulbs. Using an exposure meter, I reckon they are about 25% brighter. Wattage is unchanged and they don't seem to run any hotter."

Whatever bulbs you decide to use, the first step should be to install the relay in the circuit. It is essential with high wattage bulbs (without it the dashboard switch will melt in due time) but also very much advisable with the smaller ones. Wires always have resistance that eats up part of the voltage applied to them. The wiring from the battery going to the dashboard switch and then all along the engine bay to the lamps in the front is long enough to make the voltage actually reaching the bulbs drop by as much as 3 volts, reducing its light output very significantly.

The modification is rather simple. Find a customary Bosch-type relay and secure it to the inside of the front wing somewhere behind the left headlamp. Cut the wire going to the headlights and connect both new ends to the relay: the one coming from inside the cabin to the control coil of the relay, the other end going to the bulbs to the switched contact of the relay. You'll also need two additional wires: one to connect the opposite end of the control coil to the ground, and a second to connect the fourth pin of the relay directly to the battery. This last wire should be thick and as short as possible. Don't forget to put a fuse into this wire as well.

Measure the voltage accross the bulbs before and after the operation: it goes up from around 11 V to 13.5 V, resulting in a nice boost in light output.

But either overwattaged bulbs or only brighter ones, more light will certainly cause more disturbance to ongoing traffic. The best way to have more visibility without disturbing others would be the front foglights as they are much lower than the headlights, are as strong as the high beam and project roughly parallel to the road surface. However, in many countries the legislators banned this in clear weather.

What can be suggested without reservation are the dichroic bulbs manufactured by Philips, Osram and other companies: the bulb itself shows a strange blueish-yellowish reflection but when lit, it casts a very strong yellow light. Much better than the common white one in rain, fog or snow (not to mention the trendy but completely uses bulbs with blue tint), but I personally prefer them on dry roads, too. And they add a nice distinctive touch, and isn't this the reason why we drive Citroëns in the first place...?

Modifying Break rear lights

On the Break (Estate) models, the rear combination lights are smaller than on the Berline (Hatchback), lacking the independent stop lights. Instead, the tail and stop lights share the same bulb, the second one being brighter. If you would prefer these important lights to be separate so that people driving behind you can see better when you brake, all you have to do is to swap the function of the original stop lights and rear foglights. This is best and most easily done by swapping the wires leading to the rear combination lamps. You don't change anything else in the wiring, so there is no need to think about appropriate fuses, etc.—everything else remains the same.

The lights are originally as follows:

The modification yields the following:

After opening the tailgate, locate the two screws holding the left combination light. Undo them and remove the lens, freeing it from the positioning lug at the base. Then remove the whole light assembly by undoing the securing screw. Locate the flat ribbon cable coming from the side of the car and going into the light assembly. Wires #1 and #5 should be cross linked. The simplest way would be to separate them from the other wires, cut and reconnect them. A less destructive solution is to find a similar plug and connector in a salvage yard (7-pin on post-86 cars and 6-pin before that) and build a small additional cable with the necessary wires swapped. Then the modification can be undone at any time by simply removing the additional cable.

The cable coming from inside the car reaches the left combination lights first and then goes over to the right side. As we change the wiring before the left lamps, the changed meaning will go through the other side automatically, so we don't have to modify both sides.

From the same modification cable you can detour to the third stop light, if fitted.

Headlights warning beep

Many general car accessory stores and gas stations sell such tiny units (they generally look like a tiny beeper with two or three input wires). Naturally, the package describes how to install them properly.

However, you can build your own, too. There are many simple circuits to this end, but here's one I liked the best, mainly because of one reason: when your car is left alone, this circuit is switched off completely while many other solutions draw some (albeit small) current for their switching transistor. This is not really important while the circuit is operating perfectly, however, if the switching element becomes shorted, it will blow the fuse or flatten the battery.

You need a standard (so-called Bosch type) relay as those used in many places in a BX (cooling fan, electric windows, front foglight, etc). They are cheap and very easy to get. When the ignition is on, the relay is always switched on (it can withstand this, the cooling fan and other relays do the same). Once the ignition is turned off, the relay releases its contacts which close the way from the headlights to the buzzer.

For a buzzer, you have several options. Either buy a simple piezo buzzer at a common electronics parts store (choose one with an integrated oscillator which works only with the 12 V connected) or buy a second relay and use it as a buzzer. Granted, this is a nasty abuse of a relay but it won't suffer too much as you will turn off the headlights within a few seconds. And its sound is not that musical, either.

Retrofitting the front foglights

You will have to buy several parts, some of them new, some possibly from a salvage yard. First of all, you will need the lamps themselves. Cibie/Valeo manufactures the original (OEM) lamps used on AX, BX and Series II CX. Be prepared for the price they ask for them... If you are fortunate enough to find them in a salvage yard, examine the mirror behind the bulbs carefully: if they are already worn, leave them alone.

The lamp assembly sits in a plastic housing that snaps into the front bumper. All bumpers are born equal, there is no need to buy one from a TZx or GTi: just saw off three grille bars on each side. The lugs holding the plastic housing are already there.

You'll also need the switch for the dashboard (it's originally located in the panel above the casette holder, two blind panels cover the opening for both the front foglight switch and the switch of the electrically operated door mirrors). You can also find the multiwire connector attached to it in a salvage yard (the connectors are the same for this switch and the four others in the instrument panel facia: rear window demister, rear foglight, hazard lights, rear window washer/wiper; so if you find one of those it will do nicely). Without this connector you can use individual spade connectors but number or color code them to avoid trouble if you have to remove the switch later.

If you cannot find the lamp units or find them prohibitively expensive, shop around for universal foglamps. Raydyot, for instance, manufactures high quality lamp units sold for the fraction of the Cibie price, some of them fit quite nicely into the BX bumper (with or without the original BX plastic housing; in the second case, secure the lamps to the body parts behind the bumper, not the bumper itself). You may also find a switch from a different car make or even use a simple tumbler switch. Some foglamp kits come with a switch and relay included.

To protect the switch from the high current used by the lamps, you'll also need a standard Bosch-type relay, possibly with a socket from a salvage yard (many such relays are used in the BX, use the connector of any of them). And finally, have a 20 A fuse and a fuse holder ready.


The diagram we present is practically equivalent to the original factory setup, with two notable differences. The first is the fuse. Citroën, for unknown reasons, decided to omit the fuse in the foglight circuitry but you should never, never install any electrical appliance in your car without the proper fuse. Period. By the way, the headlights are also unfused by birth, so you could take this opportunity and install the appropriate fuses there, too (however, you should fit them inside the cabin where there is still one wire; in the engine compartment this feed is already forked into two wires so you would have to fit two fuses there, although this setup also has its advantages).

The second difference is in the layout of the earthing. The original setup grounds the right foglight behind the battery while the left one goes back into the cabin, to the main board. The reason for this escapes me. You can simply use either of the grounding methods shown on the diagram: either earth both lamps with a wire routed back to the main earthing point behind the battery, or just make a short detour behind the left headlamp to go down to a ring terminal secured under the horn supporting bracket bolt. However, for this second solution you'll have to remove the bumper, while in the first case you don't have to (not even to install the lamp units).

If you want the modification to look absolutely original, look for 20-30 cm of the wiring harness tube—those black plastic tubes, cut along the side, that house the wiring harnesses everywhere in the engine compartment. Apart from all these, only standard odds and ends like wires, spade and ring terminals, insulating tape are all you need.

In the engine compartment

Remove the battery to avoid accidental shorts while you install everything and to gain access to the wiring that goes beneath it.

Remove the black pastic grille in front of the radiator (it is secured by three screws). This clears enough room to route the wiring inside harness D (see the diagram), located under the upper edge of the bumper. Crimp or solder spade terminals to the wires, connect them to the right foglamp and fit the lamp and the housing into the bumper. Make the detouring wires for the left lamp and connect them similarly.

You can secure the relay to the inside left wing, in front of the suspension sphere. On the side wall you can find holes perfectly suited to secure the relay with a screw. Route the wiring of the relay in a harness tube (C). You may cut a small opening on the tube to accommodate the fuse holder and the fuse, as illustrated on the diagram.

In the cabin

You have to go through the bulkhead with the wire placed into the wiring harness A beneath and behind the battery. Find a few feet of rigid wire and squeeze it through the rubber grommet where all other wires cross the bulkhead (there are three holes around there, one for the wiring harness, one for the accelator pedal cable and one for the speedometer cable). Take care not to damage the existing wires or the grommet—and be prepared, it might be quite difficult to get that wire through. When done, attach or solder the new electric wire to the end of the rigid wire and pull it through the opening. You might want to add another wires later as this is the logical place to travel between engine and cabin, so do yourself a favor and pull two wires through the opening: the second one can be only a small piece just crossing the bulkhead not connected to anything; just leave it there to help you the next time you need it.

While still out of the car, join the wires of the switch multiconnector as shown on the diagram. Finally, you'll have three wires leaving the connector, one connects to the wire coming from the engine compartment (don't connect this yet), the other two tap into the appropriate wires beneath the dashboard. The feed and the return of the bulbs in the facia switches (which glow in the night when the headlights are on so that you can find them easier) goes up and down right behind the foglight switch, stretching between the mainboard (the one with the fuses and relays) and the instrument panel surroundings. Locate those wires in the wiring spaghetti and tap into them as indicated on the diagram.

With the wire going through the bulkhead still not connected, check the functioning of the switch. When you turn on the sidelights or headlights (the foglights are allowed to work only when the front lights are on), the switch should glow and when you switch it on, it should light up brightly—even if the rest of the circuit is not yet in place. If it doesn't work, check the wiring and the switch before you go on.

Finally, connect the last wire and check that your new foglights operate perfectly. Put yellow bulbs into the foglights even if you don't like them as your headlight: in rain, fog and snow they are much better.

Xenon headlights

You won't find any such replacement headlight unit for the BX. Xenon bulbs are not simply bulbs to replace traditional halogen bulbs with but a completely new technology requiring a special controller operating at very high voltages. As such, they are in no way compatible with halogens, neither electrically nor physically.

In addition, High-Intensity Discharge (HID or Xenon) units are designed to be low beam only (all HID-equipped cars use regular halogen bulbs for the high beam because it takes some time before the discharge reaches the steady phase). As a consequence, you cannot use HID-lamps in single headlight units fitted with H1 (double-filament) bulbs originally. On the other hand, to the XM, you can fit a Xenon replacement.