Jaguar XJ6 (X300) Immobiliser Problems, Won't Start
When the check engine lamp goes out a few seconds after the bulb check, the engine management has been told not to start. I'd look first at a security module that has forgotten the key after a long flat battery, then the key reader, water in the engine computer's plug, a sticking ignition switch, a boot or door switch the system reads as open or tampered with (often with the selector locked in Park), or a newly cut key that was never programmed.
How bad is it · Safe to drive
The car is refusing to start, not breaking
Nothing is damaged while it sits. The cost to avoid is guesswork: batteries, sensors, even a reader coil that measures perfect, bought before the real fault is found.
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 security module has forgotten the key
The security and locking module in the boot stores the codes of the keys it accepts. After months with a dead or disconnected battery it can lose them, and a good key is refused.
A long lay-up with a flat battery, or a rewire with the battery out. The fob may or may not work; a spare or valet key sometimes still starts it.
Try every key, valet key included. If none is accepted, the module needs dealer equipment to relearn the keys. Establish the history first: a sudden refusal and a refusal after weeks with the battery off point to different parts. If the fob only chirps and doesn't actually arm the alarm, that also points at the module.
Dealer reprogramming, or a used module that comes with its own keys.
The key reader under the dash
A coil round the ignition lock reads the key's chip and passes it to a small reader module beside the column. The coil can measure fine while the reader module behind it has failed.
Intermittent at first — refused one start in five, cured by reinserting the key — then more often, then for good. P1621 stored in the engine computer.
Measure the coil, then try a known-good reader module. The chain runs from the key's chip to the coil, the reader module, the security module and only then a coded start signal to the engine computer, so check the coil for continuity and whether the starter relay gets its signal. Turning the key on before the chip has been read can also give a refusal.
A used reader module, about £25; it can work without reprogramming. An intermittent one can be lived with by taking the key out and reinserting it.
Water in the engine computer's plug
The engine computer sits low in the right-hand footwell. Water from blocked sunroof drains runs down the pillar into its plug, and the pin that completes the immobiliser hand-off can corrode.
A car that stood outside or through a wet season; damp front carpets; sometimes the rev counter doesn't move while cranking. A computer whose pins were cleaned can work once or twice and then stop giving the start signal again.
Clear the sunroof drains first. Then take the computer out (it's behind a riveted anti-theft strap), open the plug and look at every pin.
Repair the corroded or broken pins. If that doesn't hold, a used computer: one of the same family can go straight in with no reprogramming (for example LNA1410YD/211 in place of LNA1410BF/211 on a 1995 3.2), though be ready for dealer programming if it's refused.
A sticking or worn ignition switch
The small switch behind the lock barrel also moves in and out with the key — that's how the car knows the key is present. If it sticks, the system thinks the key is still in, or the engine still running.
It depends on how you handle the key: it starts with a slow turn, or after working the key in and out; wiggling the key can stall the engine.
Take the barrel out (key to I, press the detent pin) and make sure the switch springs in and out freely.
Clean and free up the in-out movement; worn rotary contacts need a new switch.
Wiring the security system reads as tampering
The security system watches switches and circuits round the car. An open boot switch, an unplugged connector or a corroded loom looks like a break-in, and it keeps the starter disabled — even on a North American car with no key chip at all.
Other security oddities: the fob unlocks but won't lock, locking gives a warning beep instead of the normal tone, the alarm goes off on its own, the airbag lamp lights at START. Wiggling the selector or locking and unlocking sometimes clears it for a while. Recent work on the console, boot or body.
Lock the car and listen: a warning beep means the system thinks a door, the bonnet or the boot is open. Then look at the boot-lid microswitch (it has a resistor built in, so a poor contact reads as forced entry), unplugged connectors under the centre console, and the loom where it passes the body near the fuel filler.
Re-secure or replace a loose boot-lid microswitch (LNA6242AC), or refit one that has fallen off its mount; reconnect what's unplugged, for example a four-way plug under the console; repair a burnt or corroded loom.
A boot or door switch stuck at open, selector locked in Park
The security system won't arm while it believes the boot or a door is open, and in that state it keeps the starter cut and the selector held in Park. A boot-open switch that has failed, a broken wire in the loom across the right-hand boot hinge, or a sticking door latch switch is enough to cause it. A microswitch on the selector also tells the body processor whether the lever is in Park.
The boot-open or door-ajar warning stays lit with everything shut. The fob gives a single bleep and won't lock; the interior lamps may stay dark on unlocking; puddle lamps and headlamps can misbehave, and a scan tool may not even see the engine. It can come and go: a jolt over a pothole can bring the boot warning on.
Note which warning is lit. For the boot, open it with the key and shut it firmly, then unplug the boot-open switch behind the boot-lid trim: if the interior lamps come on and the engine cranks straight away, that switch is the culprit. Then look for broken wires where the loom crosses the right-hand boot hinge. For a door warning, work every latch.
Unplugging the faulty boot-open switch gets the car going at once, though the alarm then stops watching the boot until the switch is sorted. A sticking door latch switch can be freed with penetrating spray in every latch and striker, then opening and shutting each door several times.
A new key that was never programmed
On cars with the immobiliser every ignition key carries a transponder chip, and the security module has to be taught its code. A key cut on a genuine blank turns the lock but won't start the engine until it's programmed.
The new key opens the doors and turns the ignition, but the starter does nothing. Start on the original key, switch off, and the new key then works for a moment — the module is still reading the old chip.
Try the new key on its own, with the original well away from the lock. UK and other non-North-American cars have the chip (a small plug on the key head shows it); North American 1995–97 keys have none.
Dealer programming, a couple of minutes' work; or an auto locksmith who cuts the key and clones the chip, for example about £45. The valet-switch routine programs fobs only, never keys.
A failed security module or body processor
The security module in the boot and the body processor work as a pair. When one of them fails, the fob, central locking, boot release and the start signal can all die together.
Good battery voltage and fresh fob batteries change nothing. The fob's LED flashes but the car doesn't answer; the metal key opens only the driver's door; the warning lamps light but the engine won't crank, and the selector may stay locked in Park. It can follow a lightning storm close by or a jump-start that went wrong. P1621 may be stored.
The security module sits in the boot behind the left-hand trim on the filler side, with four connectors. Unplug and clean them first, especially the one beside the 26-way plug.
Replacing the alarm module and the body processor; a dealer test with a body processor from stock shows quickly whether the engine will then turn. A replacement module has to be programmed to the body processor and the engine computer by a dealer: an unprogrammed used module leaves the car completely immobilised, with no check engine lamp at all, and refitting the original brings it back.
A confused module that a reset clears
Sometimes the security system is simply in a bad state. A full reset can clear it — but if it comes back, one of the faults above is underneath.
The fault comes and goes; nothing else is wrong. Opening the car with the metal key instead of the fob, or holding the key turned in the door lock too long, can leave the system locked out for about 30 seconds; so can a weak fob battery.
Lock and unlock and try again; then the double lock and unlock with the metal key; then disconnect the battery for a few minutes (or clamp the two cables together for half an hour). Have the radio code ready.
Worth trying first, but no cure: lock and unlock and try again, the double lock and unlock, or unlocking with the fob and waiting at position I before starting. Turning the key back to 0 and trying again gets some cars going.
The inertia switch has tripped
The crash switch sits behind the kick panel at the foot of the A-pillar. A jolt can pop its button out, and until it's pushed back in the car may not crank.
No crank after a jump-start, a knock or work in the footwell; the fuses check out.
Find the switch behind the kick panel and press the big black button firmly back in.
Worth checking before anything is unplugged: pushing the button home can be all it takes. If it isn't, try another key, then meter the security module's start signal with a wiring diagram and a helper turning the key.
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.
Confirm it's the immobiliser
Key to II without cranking. Lamp goes out after the bulb check — immobilised. Stays on — it isn't; see the no-crank page.
freeCharge the battery fully and try every key
Valet key included. Low voltage alone can upset the system. A newly cut key that only starts the car straight after the original has run it hasn't been programmed.
freePress the inertia switch button in
After a jump-start or a knock, push the button on the crash switch at the foot of the A-pillar firmly home.
freeTry the reset
Lock and unlock and try again; then the double lock and unlock with the metal key, then a battery disconnection. If it works but comes back, keep going.
freeWork the key in and out; turn it slowly
If that changes anything, suspect the ignition switch.
freeLock the car and listen
A warning beep instead of the normal tone means a door, bonnet or boot switch reads open. Check the boot-lid microswitch first. If the boot or door-ajar warning is lit with everything shut and the selector won't leave Park, unplug the boot-open switch behind the boot-lid trim and try again.
freeCheck for water
Damp front carpets, blocked sunroof drains — then the engine computer's plug.
freeThen the key reader, then the security module
A used reader first: it's cheap and may need no programming. Clean the security module's four connectors before condemning it. Relearning keys, or fitting another module, needs dealer equipment.
used parts, or the dealerWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ6 (X300) Won't Crank, Just ClicksIf the check engine lamp stays lit, the causes are different.
- Jaguar XJ6 (X300) Cranks But Won't StartIf the check engine lamp stays lit and the engine turns over but won't fire.
- XJ6 (X300) fuse boxes and relays1996 and 1997 from the same page.
- Jaguar radio code generatorFree. For when the radio asks for its code after a battery reset.
- Jaguar XJ6 (X300) Key Fob Not Working, Fob ProgrammingIf the car starts but the remote won't lock, unlock or program.
- Jaguar XJ6 (X300) Steering Column and Memory Seat ProblemsIf unlocking with the remote also moves the driver's seat.
- Jaguar XJ6 (X300) Alarm Keeps Going OffIf the alarm also keeps going off after locking.
- 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.