Both Lambda sensors gone!

[krisby]

I had suspected that my front lambda sensor was failing as the readings for it are slow to change, so I've just handed it over to my mechanic to sort this afternoon, he has called and agreed the front one was slow to heat up and change voltage. But he said the back was is not working at all!

I can understand no emissions light if the front is failing, it is still giving readings, just slowly. But if the back is no working then I would expect some warning, unless someone has disabled it in Vagcom, I still haven't plugged it in yet, so maybe tonight.

I've just booked a holiday so I'm skint, so I said to proceed with the front one for now, and I'll get the back one done later on.

But what performance hit am I taking with the back one not working? That is only monitoring emissions I thought, and the front one actually detects unburnt O2 and tells the ECU to adjust fuelling to suit, thus I should possibly notice a difference in either performance or fuel economy, or both.

 

[Slimy]

The back one only monitors cat efficiency, it has nothing to do with fuelling as far as I know. You should have still had an ECU light if it had failed though?

 

[krisby]

that's what I thought too. And I had noticed in Torque that I was only getting the 1 O2 reading, as expected, but this morning I noticed there was a readout giving the CO2 reading, which I assume comes from the rear sensor, and this was showing high 290s, I only noticed it just before I got into work and didn't really think much of it as figured it was some calculation rather than an actual reading.

However, if it was a real reading, then it is high, and I guess does point to the front O2 sensor having failed, and not the rear sensor. Or it has failed and is giving a constant reading of 294g/l or whatever it is. Either way, something iffy, so I'll keep an eye on it tonight, and may plug into Vag-com as well, assuming my new cable works.

So, no dash warnings, but the wifes 1.8 20V which I also suspect as having a faulty O2 sensor as it is a constant .4v, also does not flash up any alerts, either dash or in Torque. Perhaps the O2/emissions failure are not to be trusted, makes you wonder how many cars may be running at less than optimum as no error is thrown.