Hi all,

New member to forum. My 2004 S type V6 shows random misfire on all cylinders, 9 DTCs in this ECU. Once I have cleared the codes the car will run with no engine warning light showing indefinitely. However, once the car is stopped and restarted again it will show the engine warning light within a few miles every time. my question is; how can it possibly run perfectly well for hundreds of miles and not show an engine warning light. But, mysteriously once the ignition is turned off and the engine restarted, as I said, the warning light will appear without fail. No restricted performance and car runs perfectly even when warning light is on?

 

Stop clearing the codes. Fix the fault(s).

It's not running perfectly. You're doing damage but don't realise it.

If you have ALL misfiring read the fuel trims - see the MANY MANY threads. Or look up threads with your codes.

 

That's easy. It's all part of the logic of when to turn on the light, based on the severity of the faults detected.

The most severe condition, a fault bad enough to melt down the catalytic converters, will turn on the light immediately, and cause it to flash to get your attention. This means pull over and shut off the engine. If the suspect cylinders can be identified, generally those injectors will be shut off to limit unburnt fuel reaching the cats.

Medium level faults turn on the light right away, or at least by the end of that drive cycle.

Low level faults MIGHT not be valid. To prevent a nuisance indication, the light doesn't come on until certain conditions are met. Generally this means the same fault must be seen on successive drive cycles. The faults are stored as pending, but the light only comes on once all criteria are met. You can travel hundreds of miles on one drive cycle with a pending code but no light. If the fault is still active, the light only comes on with the next drive cycle, usually after a certain amount of time or distance.

 

thanks very much but its easier said than done.the only diag. I get is random misfire all cylinders.annoyingly my ob ii reader wont talk to the ecu so no clues there. when clearing codes it says " do you want to clear emission related dtc,s in this ecu " if that's any clue?......flew through mot no advisearies no emission failures. head gasket changed 6 months ago cos of back pressure probs.doesnt empty expansion tank anymore,sorted.but I'm thinking overheating may have caused cat damage ??

 

great explanation in your last paragraph many thanks....now where do I start lol. defo reading the fault over and over, but.tester wont talk to ecu! when clearing the usual 9 dtcs only clues I get are "random misfire all cylinders..........do you want to clear emission related dtcs. ?

 

Get a better tool. They're £5 and up.

The cost of the damage is potentially the engine or the cats or ...

 

Sounds like it's time for a better scan tool. Here's what I've had for several years. It communicates with my iPhone via Bluetooth:



I run the free app that displays the data. Pretty slick stuff. Talks to my '02 with no problem. This is just one example. There are plenty of other brands out there