Jaguar XJ40 Check Engine Light, Fault Codes
On these cars the check engine light always comes with a code stored in the engine computer, and the VCM in the dash shows it. The usual culprits are the engine coolant temperature sensor (FF14), a blocked EGR port (FF39), the EGR solenoid (FF37), the oxygen sensor or its earth (FF44) and the throttle pot (FF17). I'd read the code first, with the engine off, then clear it and see whether it comes back.
How bad is it · Safe to drive
Usually safe to drive while you read the code
Most codes leave the car running normally. A blocked EGR port can make the engine ping under load, and a coolant sensor fault makes it run rich and start badly — don't leave those for months.
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.
The engine coolant temperature sensor or its plug (FF14, code 3 on early cars)
Two sensors sit on top of the thermostat housing. The brass one with a single spade feeds the gauge; the one with a two-pin plug talks to the engine computer, so a normal gauge proves nothing about the code.
FF14, or code 3 on 1987–89 cars; often after a long idle in traffic; hard starting and rough running until warm.
Unplug the two-pin sensor and measure it: roughly 200 ohms hot, over 2,000 ohms cold. Check the plug for corrosion and a bent splice. Don't wrap a new sensor in sealing tape — it earths through the housing.
Clean and reseat the plug, repair the splice, or fit a new sensor. If the code returns, check its wires at the engine computer: pin 2, blue/yellow, in the yellow 25-way plug, and pin 3, black/yellow, in the black one.
A blocked EGR port behind the throttle (FF39)
A temperature sensor near the throttle body checks that hot exhaust gas arrives when the EGR valve opens. Carbon from the breather blocks the port in the manifold, the sensor sees no rise, and the computer sets FF39.
FF39 with the car otherwise running fine; a throttle body crusted with carbon or sludge.
At idle, push the EGR diaphragm up by hand. The engine should stumble or stall; if nothing changes, the pipe or port is blocked. Also check the vacuum hose from the valve to its solenoid.
Take off the throttle body, clean the plate and bore, and clear the port with a stiff wire and a vacuum. Set the base idle afterwards. If the diaphragm won't hold vacuum, fit a new EGR valve.
The EGR solenoid or its plug (FF37)
The EGR vacuum solenoid lives beside the hot manifold, and heat kills its coil. FF37 is an electrical fault on that circuit — dirty plug, open coil or broken wire.
FF37 comes back as soon as it is cleared, sometimes straight after switching on.
Clean the solenoid plug and clear the code. If it returns, measure the coil across its two pins. The air pump solenoid is the same part — swap them to prove it.
Clean the plug first. Replace the solenoid if the coil is open; a used one may test fine cold and fail hot, so buy new.
The oxygen sensor, its heater or its earth (FF44, code 4 on early cars)
The computer only trusts the oxygen sensor once its heater has warmed it. A loose heater earth keeps the engine running on its rich fallback map; the sensor itself earths through the exhaust pipe it screws into. A sensor that really has failed makes the idle jerk.
FF44, or code 4 on 1987–89 cars, a few minutes after the engine warms up; rich running.
With the engine running, check 12 V at the two-wire heater plug, and check the heater earth eyelet on the intake manifold stud. On 1994 cars the heater relay is the yellow-base one on the bulkhead. Watch the signal swing between about 0.1 and 1 V when warm.
Tighten and clean the heater earth, give the sensor its own earth wire, or fit a new sensor.
The throttle pot: worn, oily or badly set (FF17, FF4)
The throttle position sensor tells the computer how far the throttle is open. Oil running down the throttle shaft into it gives erratic readings; a dirty plug drops the signal out.
FF17, often on the first cold start; or FF4 with stumbling, hunting idle and loss of power on hills.
Measure the output: a smooth rise from about 0.5 V closed to about 4.8 V wide open, with no jumps. Clean the plug and add a dab of dielectric grease.
Clean the plug; if the reading jumps, fit a new sensor and set it to the closed-throttle voltage.
Ignition: distributor rotor and cap (FF22, FF23)
A rotor that leaks spark to the shaft, or a worn cap, leaves cylinders misfiring. The oxygen sensor then reads the unburnt fuel and the computer flags rich, or the car simply won't fire.
FF23 (rich) with a fine-looking engine, or FF22 with a car that cranks and won't catch after a short hot stop.
With the tachometer showing cranking speed, look at the rotor and cap for cracks and tracking. Check HT lead resistance while wiggling them.
Fit a new cap and rotor; keep spares in the boot.
On the V12: unmetered air from a badly fitted air filter (FF13)
On the XJ81 V12 each bank has its own air filter. A filter box that isn't seated lets air in that the computer can't see, and one bank runs out of step.
FF13 with the light on all the time and one bank sounding breathless.
Look at how both air filters sit, then the vacuum hose to the computer where it passes through the bulkhead.
Refit the filter properly; straighten or reconnect the vacuum hose.
The engine computer itself
Rarely, the computer misreads a good sensor. Only blame it once the sensor, its plug and both wires measure right.
A code that survives two new sensors and a clean harness, with the readings in spec at the plug.
Measure the circuit at the computer plug with the computer disconnected.
Fit a used computer with the same part number.
No code on the VCM: wrong routine, or a loose plug
Codes only show with the engine off and the key at position II. 1988–89 cars keep the code only until the ignition is switched off. A light with no code at all can be a wiring or earth fault.
The VCM shows an asterisk or nothing, or the light stays on with no number.
Prove the VCM works: unplug the intake air temperature sensor or the two-pin coolant sensor and a code should appear. Then clean the engine computer's earths on the intake manifold and the bulkhead.
Worth trying: reseat a loose sensor plug, clean the earths, and read again with the engine off.
FF69 at start: the gear position switch
The computer expects to see Park or Neutral while cranking. A selector that isn't quite in Park, a worn rotary switch or low battery voltage sets FF69.
FF69 now and then, often after starting from a selector nudged out of Park.
Clear the code and see whether it returns. Watch the Park signal at the switch while moving the lever.
Worth trying: adjust the selector cable or the gearbox rotary switch, and make sure the lever is fully in Park.
FF67: the air injection relay or pump
Later cars run an air pump after a cold start and check its effect on the oxygen sensor. The light blue relay with a red base on the bulkhead is the usual weak point.
FF67, often after a cold start or in traffic.
Check the relay and its socket; the pump clutch should pull in after a warm start. The relay has a diode between pins 85 and 86 — use a replacement with the diode built in.
Worth trying: clean the relay socket or fit a new relay. FF67 may need a battery disconnect to clear.
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.
Read the code with the engine off
Ignition off for ten seconds, then to position II without starting, then press the VCM button. On 1988–89 cars read it before switching off, or it's lost.
freeNote it, then clear it
With the ignition on, pull the instrument pack fuse in the centre console panel until the gauge needles drop and then sink a little further, then refit it — this keeps the radio code. The right fuse is the one that drops the needles. Or disconnect the battery for 30 seconds, ignition off.
freeDrive and see if it returns
A code that never comes back was a passing fault. One that returns at once is a live circuit fault.
freeClean the sensor plug for that code
Coolant sensor, EGR solenoid, oxygen sensor, throttle pot or airflow meter, whichever the code points at.
freeMeasure the sensor
Resistance or voltage against its expected range before buying anything.
freeTest the EGR by hand for FF39
Lift the diaphragm at idle; no stumble means a blocked port.
freeCheck the computer's earths and plugs
When codes wander or reappear after a new sensor.
freeThe 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, 1989 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.
These positions are not on the drawings above — find them on the legend printed on the lid of the box itself.
What it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ40 Rough Idle and StallingIf the light comes with a hunting idle or stalling.
- Jaguar XJ40 MisfireIf FF23 or FF44 comes with a misfire.
- Jaguar XJ40 Transmission Problems, Limp ModeIf the transmission warning light is on and it's stuck in third.
- XJ6 (XJ40) fuse boxesWhich centre-panel fuse feeds the instrument pack on your year.
- 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.