Xsara HDI - ECU fault light, immobiliser, and keyfob battery

This is the Forum for all your Citroen Technical Questions, Problems or Advice.

Moderator: RichardW

Post Reply
User avatar
Mad Ax
Posts: 26
Joined: 10 Apr 2006, 13:40
Location: Lurking among the gothic shadows of Bath
My Cars:

Xsara HDI - ECU fault light, immobiliser, and keyfob battery

Post by Mad Ax »

OK bear with me, this is a long story!

Last year, the ECU light appeared on the dash of Mother Dearest's 2000 Xsara HDI. A trip to Citroen revealed a "Low Voltage" fault code and an "Engine Immobiliser" fault code. The Citroen mechanic cleared the faults for the princely sum of £50 and said to return the car if it happened again. How's that for diagnostics? :? I have the engineer's sheet, which says faults were cross-checked with the airbag ECU and interior ECU and no other faults were logged.

Anyhoo, the Old Man told me he'd used the car to jump-start his tractor, without the engine running. We assumed that the voltage drop was probably caused by jumping the tractor, and thought no more of it. However, a few weeks later, the light came back on.

It's been on ever since, and hasn't caused any problems.

HOWEVER, yesterday it wouldn't start. I tried several times, it would turn over and over, but not fire. Then, 2 minutes later, it started no problems. When I arrived at my destination, the doors wouldn't lock off the keyfob.

NOW... Here's where my confused and drink-addled brain starts spinning out all sorts of theories. Here is my best theory:

Someone tells me that the Xsara has a transponder in the keyfob which tells the car when it can start. I'm guessing if the keyfob battery goes flat then the engine won't start, because the transponder can't send a signal to the car. This would also explain why the doors wouldn't lock when I reached my destination.

I'm also thinking, if the transponder has any sense, it will tell the car when the keyfob battery is going flat. This could be recorded as an ECU fault re: low voltage and immobiliser fault. In theory, any sensible Citroen technician can recognise this, replace the keyfob battery, and charge £50 for resetting the fault code (and not kick the owner out saying "bring it back if it happens again").

The only uncertain part of this theory is: How come the car starts fine with the master key, if the master key doesn't have a battery in it? I tried yesterday, with the remote fob safely locked in the house 40 yards away, and it started fine.

Please feel free to shoot my theory down in flames and offer an alternative.

Finally, if I replace my keyfob battery today, how likely am I to find someone to reset the ECU fault codes FOC so that I can prove it's been fixed?
Owner of The Blue Racer - a slow and dull 2000 Xsara HDI Estate SX in boring blue
nick
Posts: 1079
Joined: 14 Mar 2001, 01:49
Location: Market Rasen, Lincolnshire
My Cars:

Post by nick »

My understanding of the way this system works is that the transponder immobiliser and the remote locking plip are two separate functions within the key. One not working will not affect the other.

The remote locking relies on the battery in the keyfob to work, the immobiliser doesn't, and should still work with the keyfob battery flat or removed. Thats why the spare/master key doesn't need a battery.

The car does have some way of knowing if the remote plip battery is going flat. On mine I think it flashes the alarm led which is inside the hazard warning light button, but shouldn't actually bring the ECU warning light on as far as I know.

I hope that helps & doesn't confuse the issue even more !
User avatar
Mad Ax
Posts: 26
Joined: 10 Apr 2006, 13:40
Location: Lurking among the gothic shadows of Bath
My Cars:

Post by Mad Ax »

Thanks for the info Nick, that's cleared up a few grey areas (hopefully!)

I don't mind the ECU light being on as the car seems to run OK and doesn't feel like it's in emergency mode (the fans dont' come on or anything). It's just that it will affect the resale value if I trade it later in the year.

I got the fob battery replaced just a few minutes ago and didn't even have to re-sync the fob, it seems to work just fine :)
Owner of The Blue Racer - a slow and dull 2000 Xsara HDI Estate SX in boring blue
nick
Posts: 1079
Joined: 14 Mar 2001, 01:49
Location: Market Rasen, Lincolnshire
My Cars:

Post by nick »

I think the system will eventually put the warning light off of its own accord if it detects the fault has cleared. (Someone told me it does it after the 12th time the car has started since the last time it detected the fault but don't quote me on that!)
So, if the flat battery in the keyfob was the cause of the light coming on it should at some point put the warning light off on its own.
Post Reply