Lambda O2 sensor questions

So my car has started rough running at low throttle, dropping a gear and flooring the pedal gets it to higher revs when it runs OK.

I assumed the issue was lambda sensor 2 (rear one) so I changed it, car still running rough. I've borrowed an OBD reader and with the car idling I'm getting the traces below.



So sensor 2 is new. Questions:

Why do the sensors register different voltages?
Any idea why sensor 2 isn't registering O2


If you plug a lambda sensor in and turn on the ignition, should it register an O2 level, and if so what?

Why do you think there is anything wrong with these traces? Both are showing O2 levels. Without knowing engine speed and temperatures, I cannot comment on whether these are the right levels but I think you need to read up on what you should be expecting as it appears to me that you do not seem to be interpreting the results correctly. Lambda sensors always return different voltages. Think about it for a second. There is a catalytic converter between the two!

Furthermore I doubt if the problem is related to the lambda sensor as they are usually not a major contributor to engine control at low throttle openings.
I think your problem is either due to a faulty engine temperature sender or an air leak into the inlet manifold as this problem is only at low throttle openings which is an idle control issue where incorrect temperature information or air leaks will have a major effect.

Besides, your first guess was wrong so it isn't the lambda, is it?

Unfortunately i didn't have access to a reliable OBD reader when the problem started, and assumed ( incorrectly as it turnd out) that the lambda sensor was the issue as the symptoms were similar to those on a previous car where a failed lambda sensor was definitely the issue.

From what you are saying it looks like I've misunderstood what the lambda outputs indicate - I had thought the % figure indicated O2 content, but now realise the voltage indicates O2 content, high voltage = low O2 and low voltage= higher O2.

So whilst under the car this evening I discovered that there may be a leak in the exhaust between the down pipe and cat - it looks like the flexible section may have parted from the down pipe, although not sure how this could cause issues apart from noise. I'll check the inlet manifold for leaks tomorrow, and the temperature sensors, which I understand can be checked with a multimeter, I'll have to look up the expected readings - I assume it is a temperature/ resistance relationship although I should be able to check with my borrowed OBD reader.

This morning I added intake air and engine temperature recording to the live data and got the trace below.

From the above the engine temperature sensor appears ok - started at ambient and gradually increased as the engine warmed up. The recording above is with the engine started from cold and left to idle, no accelerator pedal applied.

What I have discovered is that the exhaust is leaking at both ends of the flexible section between manifold and the section that connects to the cat. Would this cause issues with running?

I've checked the inlet manifold as far I can gain access and no obvious leaks. Next step to remove plugs and check colour and condition.

With respect to the function of the two lambda sensors - I understand the first one on the exhaust manifold provides info to the ecu to adjust the mixture, and the second after the cat allows further mixture trimming to ensure emissions control?

Any suggestions on what to look at next gratefully received.

The reading for o2 sensor 1 is wrong. It looks like the sensor is faulty. It should be cycling up to almost 1 volt and back, not showing a near constant low voltage.
Also exhaust leaks will throw the O2 reading out, allowing extra oxygen to enter the system.

The coolant sensor is reading low. It should be at the thermostat temperature, somewhere between 88 and 92. The exhaust leak needs fixing to get the system to work properly but a low temperature sensor input will be causing the engine to run rich which is the most likely cause of your rough running at low engine speeds.

So the previous live data trace was with the engine idling, no throttle, and also I didn't leave it long enough to get up to temperature, hence why the O2 traces were flat and temperature didn't get up to normal. So I stripped the 'sock' from flexible joint and made a temporary repair with aluminium tape.

Then took the car for a run - all now appeared OK, no hesitation or poor running at low throttle.


So the above trace looks ok to me. Went to the shop, set off home and car started to play up, got home and found the temporary repair had failed - aluminium tape had expanded with the heat and was loose and no longer sealing

So I'll be organising a new exhaust front section with flexible section.

Just to close this thread out, new front exhaust section fitted. Despite my best efforts to remove the lambda sensor gradually and carefully the threads stripped, so my spare came in handy.

With new exhaust all fitted and secure the car is running fine, no hesitation and fault codes reset.