Jaguar XJ40 Bulb Failure Warning
The bulb-failure warning on these cars is usually honest: a bulb that still lights but has gone high-resistance, a loose number-plate bulb, a corroded wing repeater on UK cars, or cracked joints in the corner modules. Stop lamps that stay on or won't light have their own suspects: the switch above the brake pedal and, for the high-level lamp, its separate relay. I'd note exactly when the warning comes on, tap every lamp with the lights on, then swap the modules side to side.
How bad is it · Safe to drive
Safe to drive — but check the stop lamps and indicators
The warning itself does no harm. It's often a real stop lamp or indicator that's out, and that matters to the car behind you.
Which one applies to yours depends on your symptoms, your model year and what has already been done to it.
Likely causes, most common first
Read the check cost before the odds: a free check that rules a cause out beats a likely guess that costs money.
A bulb that still lights but fails the monitor
Four modules, one at each corner, measure the current through every exterior lamp, whether it's switched on or not. A bulb doesn't have to be dead to trip them: an aged filament, a filament that's broken and touching, or the wrong wattage is enough.
Every lamp lights, yet the warning stays on, or comes and goes over bumps, at certain revs, or only with the lights off. A broken filament that touches again gives a warning that comes and goes.
With all lights on, tap each lens or bulb sharply and watch the dash. Then change the rear bulbs one at a time, checking the warning after each. If one stop lamp is dimmer than the other, compare the wattage stamped on the two bulb bases.
Fit a new bulb of the correct wattage with a plated, silver-coloured base rather than brass. If a fresh bulb doesn't cure it, try one from another batch: a bulb can pass a meter test and still not light reliably.
Loose number-plate bulbs in their spring clips
The two number-plate lamps hold small bullet or festoon bulbs between spring contacts. Slamming the boot lid shakes them loose, and a cracked plastic clip lets the bulb lose contact now and then.
The warning comes and goes, often after the boot has been shut, with no lamp visibly out.
Take both plate lamps apart and look at the clips and the bulb ends with the lights on and the lid closed.
Bend the contacts for a firm grip, glue a cracked clip, or solder the clips to the bulb ends.
A corroded wing side-repeater lamp on UK cars
UK cars have a small indicator repeater in each front wing; US cars don't. Water gets in where the cable enters between the wings, the holder corrodes and loses its earth, and the module reads the resistance as a failed lamp even while the repeater still flashes.
Warning with the indicators, often a fast tell-tale on one side, sometimes gone once the car has cooled.
Pull the repeater out of the wing and look at the bulb holder and its earth. Wiggle it with the indicator on.
Clean the holder or fit a complete new repeater lamp; check the boot-floor earth bolt for that side too.
A bulb-failure module with cracked joints or a failed part
The front modules sit behind the headlamps and the rear ones behind the boot side trim. Inside, each lamp has its own relay on a main board with a sub-board. Cracked joints at the relays and the sub-board connector are the usual failure, especially on the early cars; sometimes a diode, resistor or capacitor has blown.
Every lamp and holder is good, the warning persists, and the fault follows the module when you swap it to the other side. A faulty front module can also kill a rear stop lamp on the same side. One stop lamp dead or flickering while that side's indicator still works points at the stop-lamp relay inside the rear module.
Swap the front pair left to right, then the rear pair. If the warning or the fast flash moves, open that module and inspect every joint with a strong magnifier.
Reflow all relay and connector joints on both boards, or fit a used module with the same part number — they vary by market, so read the label on the case.
A blown or wrong-rated fuse for one rear corner
Each rear corner's stop, reverse, indicator and fog lamps share a fuse in the footwell box on that side. No feed means the module has nothing to switch, so a whole group of lamps on one side goes out together and the warning comes on.
Several lamps dead on one rear corner at once — often after fitting LED bulbs — and the fault stays on that side when the modules are swapped. If that side's indicators and reversing lamps still work, the fuse is fine.
On 1993 cars the rear fog, reverse, indicator and stop lamps are on B8 (15 A) in the box on that side; on a 1988 car the left rear stop, reverse and indicator fuse is F7 (7.5 A). Test it live and check its rating. A fuse that tests good on a meter can still sit in dirty seats: clean them.
Fit a fuse of the rated value. A 10 A fuse in a 7.5 A slot has set Circuit 2 failure and Bulb failure on a 1988 car.
Side and tail lamps dead on 1993–94 cars: relay module DBC10009
On the last cars the side and tail lamps are switched through relay H3 in relay module H, DBC10009, with a yellow base under the passenger side of the dash. Its relay contacts and board joints wear, and the lamps drop out together.
Tail lamps, front side lamps and plate lamps all out or flickering together, headlamps fine; a knock on the knee bolster sometimes brings them back.
Pull the module and open it. If closing relay H3 by hand lights the lamps, the module is at fault; if not, check the switch signal on its yellow connector.
Clean the relay contacts and resolder the board, or fit a used DBC10009 — it's no longer made, and a home repair may only last a while.
Both tail lamps dead: burnt diodes on the light switch board
The light switch board carries diodes in the tail-lamp switching. When they burn, the tail lamps stay dark although the fuses, modules and relay module are fine.
Both tail lamps out together; module and relay swaps change nothing.
With the lights on, look for 12 V on the red/green tail-lamp wire. None, with the module relay and fuse C7 both good, points back to the switch.
Replace the burnt diodes on the switch board.
High-level stop lamp quits after a few presses, or stays lit: its own relay
The stop lamp in the rear window doesn't go through the bulb-failure modules: the pedal switch earths its own relay in the right rear of the boot. On 1993–94 cars that relay is element L1 of relay module DBC10422, on a 20-way black connector at the top front right of the boot near the aerial motor; earlier cars use a plain cube relay beside the right rear module.
The window lamp works for the first five or six presses, then quits until a restart, or it stays lit after you park. All its bulbs come and go together while the main stop lamps keep working. On 1993–94 cars the remote boot release can misbehave at the same time.
Take off the right rear boot trim and find the relay. With the pedal pressed, look for 12 V on the Green/black wire at pin 8 of the black connector, and check the Black earth at pin 15. If the lamp stays lit after parking, pull the relay and see if it goes out.
Fit a used DBC10422 on a 1993–94 car, or a new cube relay on an earlier one.
Corroded bulb sockets or a poor earth in a rear cluster
The rear holders have weak spring contacts, and corrosion on them is hard to see. The modules read that resistance as a failing lamp. Each rear cluster earths through a bolt in the boot floor under the side carpet, just inboard of the module.
Tapping a rear lens makes the warning go off or come back; one side's lamps a little dimmer than the other.
Pull each rear bulb and look at the socket contacts and the bulb base. Take out the earth bolt, clean it and the floor around it. For a dead stop lamp, measure both bulb contacts to earth with the pedal pressed: 12 V on both means a bad earth, 12 V on neither means the module or its wiring.
Worth trying: clean every socket with contact cleaner or fine abrasive, polish the spring contact, and coat with dielectric grease. The earth bolt sits in the boot floor under the trim, just in front of each rear cluster; if the bolt has stripped its hole it never tightens, and a new metric bolt cures it.
Stop lamps stuck on, dead or working backwards: the switch above the pedal
The stop-lamp switch sits at the top of the brake pedal in the pedal box, under the dash by the steering column: a pair of micro-switches sharing a mounting with the cruise-control switch. It earths the stop-lamp circuit for both rear modules and the high-level relay, so one bad switch takes out every stop lamp at once, or keeps them all lit.
All stop lamps out together while the indicators and reversing lamps work. Or the lamps stay on after you park and go out if you lift the pedal a few millimetres. Lamps lit with your foot off the pedal and out when you press it usually follow a switch replacement.
Unplug the switch connector (on early cars a 2-way with Black and Purple/blue wires) and bridge it: if the stop lamps come on, the switch is at fault. If they stay lit with it unplugged, look elsewhere. Make sure the pedal returns fully.
Worth trying: the switch plate's slotted screw holes set the point where the lamps come on, so mark the original position and adjust. Clean and grease the connector, or fit a new switch. If the normally-open and normally-closed contacts have been swapped, rewire them. A pedal that doesn't return comes first: a sticking servo has held the lamps on overnight and flattened the battery.
Bulbs with the wrong base, the wrong wattage, or LEDs
The modules judge each lamp by the current it draws. A brass or copper base adds enough resistance, and an LED draws too little, so a lamp that plainly lights still reads as failed. A bulb of a different wattage does the same.
The warning arrived after bulbs were changed or LEDs went in, and every lamp lights.
Look at the bases of any recently fitted bulbs: they should be silver-plated, not brass-coloured. Compare their wattage with the originals, and list every outside position where an LED has gone in.
Worth trying: plated-base bulbs of the original rating, and ordinary filament bulbs back in place of any LEDs.
Front fog lamps removed without dummy plugs
The modules watch the front fog lamp circuits too. With the lamps gone, the empty plug reads as a failed bulb, so the factory fitted a resistive dummy plug in the fog-lamp connector on cars without fogs.
Warning ever since the fog lamps came off, or on a car that never had them; it goes out when the lamps are plugged back in.
Look behind and below the front indicators, behind the spoiler, for the 2-way fog connector and its plug; check the plug's leads for corrosion.
Worth trying: fit resistive plug DBC11578 on each side, or a suitable resistor. Don't pull the fog fuse instead — that brings up a fuse warning.
The order to check them in
The point of the order is money: each step either rules a cause out or makes the next one worth doing. Green is free, amber costs a little, red is the one to arrive at last.
Note exactly when the warning comes on
Lights off, sidelights only, braking, indicating, over bumps or at certain revs. Braking points at the stop lamps; a fast tell-tale on one side points at that side's indicators.
freeWalk round every monitored lamp
Tail, side and side-marker lamps, both stop bulbs per side on North American cars, indicators and wing repeaters, reversing lamps, both number-plate lamps, the rear fogs in the bumper, and the front fogs or their dummy plugs.
freeHave someone press the brake pedal
All stop lamps dead or stuck on points at the switch above the pedal: unplug it and bridge the connector. Only one side dead: earth the Purple/blue wire at pin 8 of that module's 18-way white connector, and the stop lamp should light.
freeTap each lamp with everything switched on
Watch the dash while you tap the lenses and wiggle the plate lamps. A change on a tap finds the bulb or holder.
freeClean the sockets and the boot-floor earths
Every rear socket, the wing repeaters on UK cars, and the earth bolt beside each rear module. Finish with dielectric grease.
contact cleanerSwap the modules side to side
Front pair, then rear pair. If the warning or the fast flash moves to the other side, that module is the fault.
freeOpen the faulty module and reflow its joints
Relay pins and the sub-board connector first, under a strong magnifier. Do all four if they're the early type.
a soldering iron, or a used moduleThe fuses named above — where they sit
The highlighted position is the one the procedure talks about. Pulling a fuse is a stop-gap, never a repair: check what the circuit feeds before you pull it.
Circuit descriptions from the Jaguar fuse tables, 1993 edition. Drawing by jagaux.com from the factory electrical guide. Fuse layouts change between model years — read the legend on the lid of your own box before pulling anything.
Circuit descriptions from the Jaguar fuse tables, 1993 edition. Drawing by jagaux.com from the factory electrical guide. Fuse layouts change between model years — read the legend on the lid of your own box before pulling anything.
What it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ40 Turn Signals Not WorkingIf the indicators flash fast or not at all along with the warning.
- Jaguar XJ40 Headlights Not WorkingIf a headlamp is dead or dim — the same front modules switch it.
- Jaguar XJ40 FUSE 1 or FUSE 2 FAIL WarningIf a fuse or circuit warning shows on the dash alongside it.
- XJ6 (XJ40) fuse boxesWhich fuse feeds each corner on your year — other years from the same page.
- Jaguar VIN decoderConfirm your exact model year, engine and factory options before ordering parts — free.
Ratings, fuse numbers and torque figures vary by model year and market. Read the legend on your own car before pulling anything, and use the figures in the manual for your car, not the ones in a video.
Check your VIN: exactly which car you have and which bulletins apply to it.