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engine light keeps coming on


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we bought a 52 plate ford focus in february it had full ford service history and last serviced in january, about 6 weeks after buying it the engine management light came on, took it to a garage to get it checked out, it said it was either a cat or lamba switch problem , we replaced the lamba swithch and had noproblems for a couple of weeks then the light came back on, went back to the garage and it said the same thing so booked it in and replaced the cat, again no problems for a couple of weeks then you guessed it already i expect the light came back on went back to the garage, and tobe fair to them they admited they were stumped but decided that an oil change might help so booked the car in and changed oil and filters, everything fine for a couple of weeks then yesterday the light came back on. the car seems tobe driving fine, no misfiring, starts first time everytime(problably wish i hadnt said that later) not smoking or anything .what could the problem be and how much more should i spend trying to find the problem, it's getting to the stage where i feel like just scrapping it or just leaving the light on and seeing what happens

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Every time you start your car the ecu does a system check which can take 40 mins or more. If you don't take the car for a long enough run the ecu doesn't get a chance to check all the systems. When you then restart the car it starts from the beginning again. When a fault is found its stored on the ecu's memory to be read later by the diagnostic equipment. Not all faults turn on the engine management light.

The light needs to be reset even after a part that is determined to be the cause is replaced. So when the garage replaced the 'faulty' part and reset the light they should of taken it on an extended road test to allow the ecu to do a full system check. If they haven't done that and seen that the light didn't come on after a short period of time they wouldn't know that the problem was still there, although since no light is on they assume the fault is fixed.

So after handing you back the keys with the light out you assume everything is fine. Until you take the car for a long enough run. This can be days or even weeks since you paid your hard earned cash to fix a fault that was never the cause.

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Not to add a bit of doom to this,

But I had the yellow engine light come up on my people carrier (citroen C8)

Asked Citroen about fault they said bring it in to check, The engine completely seized within a month.

Now to repair it, I was quoted telephone numbers. They just stopped short to say I require a new engine.

Be careful with this warning light. It could be a small thing or large.

I now have a Focus.

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