Low Oxygen Sensor Voltage On One Bank

[Keggers]

Hi Guys

I have a 1998 996 Carrera tiptronic in RoW spec with 89,000 miles.

I had a play with my Durametric this morning and tested the oxygen sensors (I have an Irish car so it only has pre-cat sensors rather than pre and post). I've had ongoing issues with low mileage in traffic and have been averaging 13-14mpg (UK mpg - this is about 11 US mpg) and have been chasing my tail trying to find the cause.

I recorded the values over an extended period (4 to 5 minutes, after it had warmed up and gone into closed loop) and calculated the average values for each. I got an average 0.426 volts for "O2 Sensor Voltage Ahead of Cat. Conv." and 0.356 volts for "O2 Sensor Voltage Ahead of Cat. C. Bank 2". Both fluctuated but this was the average. I repeated this several times I got similar results. [edited to add that on a subsequent test I got 0.39 on bank 1 and 0.34 on bank 2]

My understanding from reading old posts on here is that both should be averaging 0.45 volts, so both seem low but one seems seriously low. I also understand (possibly incorrectly!) that low readings will be telling the computer that the engine is running lean.

Am I correct in thinking that it wouldn't be possible for only one bank to be running lean while the other is ok (or slightly lean), which would suggest that at least one of the sensors is faulty? Would the engine be trying to compensate for the low reading from this sensor and sending more fuel even though it doesn't need it, so that it is actually running rich?

Am I being hasty in assuming that I probably need to replace one or both sensors?

In case this is relevant, the durametric told me that it is idling at 680rpm (I thought it was meant to be 700rpm) and the MAF is recording about 16-17kg/hr at idle (up to about 30 at 2000rpm and 37 at 2500rpm) with aircon switched off. Also, I've never had a CEL warning light.

Thanks for any help. Sorry for so many questions in the one post!

Greg

Edited April 15, 2010 by Keggers

[number9ine]

Hi Guys

I have a 1998 996 Carrera tiptronic in RoW spec with 89,000 miles.

I had a play with my Durametric this morning and tested the oxygen sensors (I have an Irish car so it only has pre-cat sensors rather than pre and post). I've had ongoing issues with low mileage in traffic and have been averaging 13-14mpg (UK mpg - this is about 11 US mpg) and have been chasing my tail trying to find the cause.

I recorded the values over an extended period (4 to 5 minutes, after it had warmed up and gone into closed loop) and calculated the average values for each. I got an average 0.426 volts for "O2 Sensor Voltage Ahead of Cat. Conv." and 0.356 volts for "O2 Sensor Voltage Ahead of Cat. C. Bank 2". Both fluctuated but this was the average. I repeated this several times I got similar results. [edited to add that on a subsequent test I got 0.39 on bank 1 and 0.34 on bank 2]

My understanding from reading old posts on here is that both should be averaging 0.45 volts, so both seem low but one seems seriously low. I also understand (possibly incorrectly!) that low readings will be telling the computer that the engine is running lean.

Am I correct in thinking that it wouldn't be possible for only one bank to be running lean while the other is ok (or slightly lean), which would suggest that at least one of the sensors is faulty? Would the engine be trying to compensate for the low reading from this sensor and sending more fuel even though it doesn't need it, so that it is actually running rich?

Am I being hasty in assuming that I probably need to replace one or both sensors?

In case this is relevant, the durametric told me that it is idling at 680rpm (I thought it was meant to be 700rpm) and the MAF is recording about 16-17kg/hr at idle (up to about 30 at 2000rpm and 37 at 2500rpm) with aircon switched off. Also, I've never had a CEL warning light.

Thanks for any help. Sorry for so many questions in the one post!

Greg

If there are no fault codes, then your ECU considers the sensors to be within range. I don't know whether or not you're on the cusp of values that would indicate the sensors are faulty.

Keep in mind that the O2 sensors are there to find faults with what's happening upstream, and their values might not indicate a bad sensor but a symptom of something else gone wrong. Are you up-to-date on your maintenance (oil, plugs, filters, etc.)? Do you have any aftermarket parts on the drivetrain? Are you using the minimum octane recommended in your owner's manual? Are you calculating your gas mileage at each fill-up or via your OBC? If both, do they correlate?

Are you running aftermarket wheels and/or tires in a size different from the 17" or 18" OEM sets?

Please let us know if your Durametric is finding any fault codes.

Mark

[Keggers]

If there are no fault codes, then your ECU considers the sensors to be within range. I don't know whether or not you're on the cusp of values that would indicate the sensors are faulty.

Keep in mind that the O2 sensors are there to find faults with what's happening upstream, and their values might not indicate a bad sensor but a symptom of something else gone wrong. Are you up-to-date on your maintenance (oil, plugs, filters, etc.)? Do you have any aftermarket parts on the drivetrain? Are you using the minimum octane recommended in your owner's manual? Are you calculating your gas mileage at each fill-up or via your OBC? If both, do they correlate?

Are you running aftermarket wheels and/or tires in a size different from the 17" or 18" OEM sets?

Please let us know if your Durametric is finding any fault codes.

Mark

Hi Mark,

Car had a minor service (oil, filters, etc.) only about 4 months back and had a major service (plugs etc.) 16 months ago. Everything is completely standard (apart from having a PSE), including wheels. Yeah, I've manually checked mileage and it matches the OBC very closely. Durametric hasn't found a fault code since I bought it apart from an alarm code!

I did have a vacuum leak a month or so back but that was fixed and the rest of the vacuum lines seemed fine.

Whats really puzzling me is the big difference between the two sensors.

Greg

[Richard Hamilton]

I think those reading sound perfectly normal.  The voltage should fluctuate extremely rapidly in the range 0.04 to 0.79 volts.  The only time to suspect a dodgy sensor is if the readings are outside that range, or if the voltage change is sluggish.

On DME 5.2.2 (mechanical throttle) the idle speed is 680rpm, so you are spot on there too.  Also the MAF should be reading 17 kg/hr +/- 2.5 (at idle), so that's fine too.

[wvicary]

Are your readings only taken at idle? O2 sensors can problematic but not set a code if they become lean/rich biased or stall under the allotted time period so it's best to monitor the short term and long term fuel trims on a drive cycle.