Replaced 02 sensors, engine light still on

Question

I recently bought a second hand VW Golf.

It was 'serviced' and MOTed before the guy sold it to me.

He said three 02 sensors were replaced during the servicing.

The day I drove it away, the engine light came on.

I am taking it back on tuesday, but would like an opinion from a third-party.

I am going to buy a code reader tomorrow so I can go to the mechanics knowing where to look.

Could it be that the computer hadn't been erased, and so it still thinks what it thought before the faulty 02 sensors were replaced.

It passed the emissions test (passed MOT), so I think I can safely rule out a faulty cat converter?

Answer 1

Typically, these emissions/inspections tests connect to the on-board computer, and that computer will tell you if a code has been cleared. For example, with the O2 code, I think most cars require you to drive 50 miles before the computer will give the "all clear" after the code has been cleared.

If you had 3 O2 sensors replaced, then my guess is that your down-stream O2 sensor, which is after the catalytic converter, has gone bad even though it was just replaced. This can happen if you have a bad catalytic converter: it blows junk down the exhaust and damages the sensor, even a brand new one.

Unfortunately, I'm not sure what a good way to diagnose this is, since you do not have evidence that the O2 sensor installed is actually a brand new one. I can say, though, that it is not completely unrealistic that the sensors were replaced, and one of them is now damaged again. Especially since it passed the inspection.