O2 sensor question

Hunter321

I have high CO and HC emissions on my 06 Polo. Serviced it today and changed the plugs. Hooked up the OBD reader and had a look at the O2 sensor voltages. The pre cat sensor was fluctuating between 0.1 and 0.8V which I think is normal. The post cat sensor is reading between 0.7 and 0.8V. Is this a bit high, indicating rich mixture or is this OK. Any help appreciated.

PukkaStukka

Both sensors seem fine. The post cat might be a little high, but that sensor only reports on the cat converters performance. The Post Cat sensor does not control fuelling, that's the pre cat sensors job!

Hunter321

Thanks for the reply. That's what I thought but wondering if the cat is not doing its job, causing a high post cat reading.

PukkaStukka wrote: »

Both sensors seem fine. The post cat might be a little high, but that sensor only reports on the cat converters. The Post Cat sensor does not control fuelling, that's the pre cat sensors job!

PukkaStukka

If the cat isn't doing its job properly, you'd probably see the second sensor oscillating like the first one when the car is at normal temperature. To prove the point, scan both sensors when the car starts from cold and see if sensor 2 mimics sensor 1 for a short period.

Stoolbend

Get a compression test done.

mullingar

I was asked to look at an 06 1.2 polo recently that failed its NCT on emissions. Discovered that the pre cat o2 sensor was stuck on 0.4V due to a cable fault. Now fixed.

This is a screen grab of the 2x o2 sensors now when revving the engine. I'm not sure it will pass as the exhaust smells oily. If it fails, it needs a compression check....

Hunter321

Thanks for the post Mullingar. That looks similar to mine when revving. The after cat sensor is steady at idle.
One thing I did notice, not sure if this is normal, when I rev the fuel control goes into 'open cycle engine temperature too low' for a short period and then back to 'closed cycle'. I'm not sure if this is normal, possibly.
I've put some cat cleaner in the tank and going to give it a go. These cars are a bitch for the emissions.

nd

I've a 04 polo that failed emmisions numerous times,I eventually changed the piston rings and it just passed the emmisons with very low levels. It is a very big job to change the piston rings though, more so than on another normal car as you can't undo the conrods without removing the engine( you can't do it by just dropping the sump).

But it was using at lot of oil. If yours isn't then you might not have to go to that length.

Hunter321

Thanks for the info nd. Mine is burning quite a bit of oil. This isn't a job I would take on, so if she fails again I will probably sell on.

nd

What was the actual lambda number on the fail sheet? My front sensor was not working correctly also. but replacing it was did not fix the emmisions enough but it needed to be replaced anyway. i got a new branded one for 10 euro on ebay. It wasn't specifically for a polo but just crimped the wires. Unbranded ones on ebay have never worked correctly when I've bought them.

Hunter321

The lambda reading was 0.97 from memory. That's what had me looking at the O2 sensor. But I wanted to be sure it was faulty before replacing. I was looking at them for 70/80 euro. Obviously for a tenner I would stick one anyway. Wasn't sure if these were specific to car model or not.

PukkaStukka

Hunter321 wrote: »

Thanks for the info nd. Mine is burning quite a bit of oil. .

I think that's your real problem. It's nothing to do with the cat or the O2 sensor.

All things considered, Sensor 1 is switching correctly on the basis of what you've said here.

Sensor 2 is flat line which suggests CAT efficiency is ok.

If the CAT was defective, you'd see sensor 2 switching something like sensor 1 and possibly a P0420 code flagged as well. In fact, with a lambda reading of .97, the sensor is probably trying to run the engine on the lean side on account of the extract HC its seeing from the oil burning.