The usual failure mode is that the car runs rich. But can they fail to make it run weak?
I have a regular customer with an old cavalier 1,8 injection (single cam engine) I see the car every year for a service and MoT, for the last few years it has had a splashy misfire at low speed, which has very little effect on driving it, and he doesn't find it a problem, so I have just ignored it. This year it only just scraped through the emissions on lambda. The apparent cause is weakness, since a slight increase in fuel pressure makes it run perfectly. I have hunted for an air leak without success, even took off the egr valve in case it was stuck open. So the only thing I can think is that the lambda sensor is giving the wrong info. and making it run weak.
The car has 140k on the clock, always serviced once a year and only does about 4k a year.
Any ideas (for when it comes in next year !!) I suppose I could just disconnect the sensor and see what it does ?
Mrcheerful