ABS Warning Light

Does anyone know why my ABS warning light comes on occassionally when I first start the engine. If I switch off then back on again, it goes out. I know that in the manual it states that, when lit, it means the ABS has switched itself off and to consult OPC. But then it's not on all the time, only when I first start up (stays on until I turn off the ignition then back on again) Seems to be when the engine warms up, it's ok. Could the sensor/switch have a fault? Thanks, John

It could be an ABS sensor, the brake light switch, or the MAF sensor. A diagnostic check should show up what is wrong, as a fault will be stored in one of the control modules. MAF sensor faults usually cause the PSM and ABS lights to come on under heavy acceleration, so I think it is more likely the brake light switch or a sensor fault. It might simply be water or corrosion in one of the connectors.

Thanks for that Richard. Will check it out this weekend. John

On a similar theme, my 2004 C2 exhibits ABS & PSM warning lights on a frequent basis (every journey now). The car always starts & runs OK - without warning lights - but they come on usually after hitting some sort of bump / irregularity in the road, typically while braking. The ABS is definitely turned off when the warning says it is - as I found out when exploring the braking limits one day & a front wheel locked up briefly ! The lights then stay on until the car is stopped / engine turned off & re-started. Engine running / performance does not seem to be effected while the fault is indicated & I had assumed it probably was some sort of connector problem - but have been too lazy to resolve while the weather's been so bad (no garage facilities to work on the car in the warm !) If it was the MAF sensor (which I assume is maybe more expensive than ABS sensors), are there likely to be any other indications that can be detected without a diagnostic check ? I'd like to estimate the potential costs before I let my local indie loose on the car & will work my way round all of the connections this weekend - weather permitting Thanks for any guidance, Paul 2004 996 C2 40th Anniversary

Paul - a diagnostic check should show up what is wrong. It may well be the brake switch, but a check would show if it is an ABS sensor. A sensor is £92.40 +VAT. You could try changing the switch first though - see below. Tony - the part number is 996.613.113.02 at £10.46. Fitting is easy. It is a bayonet fitting in the bracket, so just disconnect the plug, and turn the switch 1/4 turn to release it. On the new switch, pull the plunger all the way out before inserting it in the bracket, and twist 1/4 turn to lock it in. Reconnect the plug.

Richard, thanks also ! The car was connected to a Porsche diagnostic tester at an Indie at the weekend, with the fault indicating as an ABS presuure sensor. The guy with the tester suggested the brake light switch was in the same circuit & was far more likely to be the cause. Since then, I've been paying far more attention to when the warning first appears & it does appear to be very soon after the brake pedal is depressed - re-inforcing everybody's suggestion it is the BLS (although as an afterthought, a faulty ABS pressure sensor would possibly also cause a system fault at the same time). At the cost you've indicated I think I'll be buying / trying the switch first ! Regards & thanks again. Paul 2004 996 C2 40th Anniversary

I'll bet £1 to 1p that this was the fault code:

Richard, I didn't see the actual screen detail / fault code - but the technician said it was an ABS pressure sensor fault. Based on yesterday's comments I bought a BLS on the way home last night & fitted it (Definitely not easy to get your hands in between the various heater ducts etc to turn the new switch into position - I've got bruised knuckles now). Seems to be a great result - first journey this morning in 2 months without the warning lights coming on ! Update : 2 days, quite a few journey's including bumpy roads & no warning lights - issue definitely gone ! Hopefully the warning lights will now stay away ! Thanks a lot for the great information & another car fixed at low cost. Regards, Paul 2004 996 C2 40th Anniversary

Where abouts is this brake switch please ? John

It's on a bracket next to the brake pedal. To the left IIRC.