Fuel Trim /O2 sensor issue
09-19-2015, 12:10 PM
#1
AutoX
Thread Starter
Join Date: Jun 2004
Location: UK
Posts: 14
Likes: 0
Received 0 Likes
on
0 Posts
Fuel Trim /O2 sensor issue
Hi,
My '98 996C2 is running fine with no noticeable problems and no CEL however I plugged in my OBD2 tool the other day "just for a look" and although there were no fault codes stored I could see that bank 1 O2 sensor was fixed at 0.430V constantly(even during a long run) whereas bank 2 O2 sensor fluctuates up and down as expected. (this is a UK spec car with only 1 O2 sensor per side) As a result of this Bank 1 fuel trim was also fixed at 0 constantly and indicates "open loop fuel system fault". Bank 2 fuel trim constantly moves up and down and indicates "closed loop" as you would expect only changing to open loop occasionally e.g. when in overrun.
So I purchased a new 02 sensor (Correct Bosch replacement part number) and fitted it. Now the Bank 1 O2 sensor reading does now fluctuate but not as much as bank2. At idle it spends most of the time around 0.040V and occasionally changes up to as much as 0.8 during a drive. However the strange thing is that the bank 1 fuel trim is still locked at 0 and never moves no matter what - idle, accelerating , overrun. Bank 1 still indicates "open loop - system fault" .
I have attached a short excel spreadsheet showing results from a short drive . As the scanner only records every 3 seconds it doesn't show the most noticable thing which is that bank 2 O2 sensor fluctuates raplidly whereas bank1 O2 sensor only changes value occasionally.
Anybody got any ideas??? Thanks
My '98 996C2 is running fine with no noticeable problems and no CEL however I plugged in my OBD2 tool the other day "just for a look" and although there were no fault codes stored I could see that bank 1 O2 sensor was fixed at 0.430V constantly(even during a long run) whereas bank 2 O2 sensor fluctuates up and down as expected. (this is a UK spec car with only 1 O2 sensor per side) As a result of this Bank 1 fuel trim was also fixed at 0 constantly and indicates "open loop fuel system fault". Bank 2 fuel trim constantly moves up and down and indicates "closed loop" as you would expect only changing to open loop occasionally e.g. when in overrun.
So I purchased a new 02 sensor (Correct Bosch replacement part number) and fitted it. Now the Bank 1 O2 sensor reading does now fluctuate but not as much as bank2. At idle it spends most of the time around 0.040V and occasionally changes up to as much as 0.8 during a drive. However the strange thing is that the bank 1 fuel trim is still locked at 0 and never moves no matter what - idle, accelerating , overrun. Bank 1 still indicates "open loop - system fault" .
I have attached a short excel spreadsheet showing results from a short drive . As the scanner only records every 3 seconds it doesn't show the most noticable thing which is that bank 2 O2 sensor fluctuates raplidly whereas bank1 O2 sensor only changes value occasionally.
Anybody got any ideas??? Thanks
09-19-2015, 02:51 PM
#2
Race Director
Hi,
My '98 996C2 is running fine with no noticeable problems and no CEL however I plugged in my OBD2 tool the other day "just for a look" and although there were no fault codes stored I could see that bank 1 O2 sensor was fixed at 0.430V constantly(even during a long run) whereas bank 2 O2 sensor fluctuates up and down as expected. (this is a UK spec car with only 1 O2 sensor per side) As a result of this Bank 1 fuel trim was also fixed at 0 constantly and indicates "open loop fuel system fault". Bank 2 fuel trim constantly moves up and down and indicates "closed loop" as you would expect only changing to open loop occasionally e.g. when in overrun.
So I purchased a new 02 sensor (Correct Bosch replacement part number) and fitted it. Now the Bank 1 O2 sensor reading does now fluctuate but not as much as bank2. At idle it spends most of the time around 0.040V and occasionally changes up to as much as 0.8 during a drive. However the strange thing is that the bank 1 fuel trim is still locked at 0 and never moves no matter what - idle, accelerating , overrun. Bank 1 still indicates "open loop - system fault" .
I have attached a short excel spreadsheet showing results from a short drive . As the scanner only records every 3 seconds it doesn't show the most noticable thing which is that bank 2 O2 sensor fluctuates raplidly whereas bank1 O2 sensor only changes value occasionally.
Anybody got any ideas??? Thanks
My '98 996C2 is running fine with no noticeable problems and no CEL however I plugged in my OBD2 tool the other day "just for a look" and although there were no fault codes stored I could see that bank 1 O2 sensor was fixed at 0.430V constantly(even during a long run) whereas bank 2 O2 sensor fluctuates up and down as expected. (this is a UK spec car with only 1 O2 sensor per side) As a result of this Bank 1 fuel trim was also fixed at 0 constantly and indicates "open loop fuel system fault". Bank 2 fuel trim constantly moves up and down and indicates "closed loop" as you would expect only changing to open loop occasionally e.g. when in overrun.
So I purchased a new 02 sensor (Correct Bosch replacement part number) and fitted it. Now the Bank 1 O2 sensor reading does now fluctuate but not as much as bank2. At idle it spends most of the time around 0.040V and occasionally changes up to as much as 0.8 during a drive. However the strange thing is that the bank 1 fuel trim is still locked at 0 and never moves no matter what - idle, accelerating , overrun. Bank 1 still indicates "open loop - system fault" .
I have attached a short excel spreadsheet showing results from a short drive . As the scanner only records every 3 seconds it doesn't show the most noticable thing which is that bank 2 O2 sensor fluctuates raplidly whereas bank1 O2 sensor only changes value occasionally.
Anybody got any ideas??? Thanks
That it can only read the values every 3 seconds is in itself a red flag.
What you should see is both O2 sensor readings fluctuating between 0.1V to 0.7V at idle at about once a second. The DME constantly varies the air/fuel mixture from just a bit too rich to a bit too lean to know that it has control of the fueling. In the lean case this supplies oxygen to the converters which store this and use it to process exhaust gases. You can't see this of course because the car isn't equipped with the sensors after the converters.
At higher RPMs the DME varies the fuel mixture more often maybe several times a second.
At this rate you would need an analog scope to constantly monitor and view the voltage emitted by the sensors.
Absent a CEL and O2 sensor related or fueling related error codes, and absent any pending error codes, I'm thinking your scanner just is not up to the task and you are chasing what appears to be a phantom problem.
09-19-2015, 04:45 PM
#3
AutoX
Thread Starter
Join Date: Jun 2004
Location: UK
Posts: 14
Likes: 0
Received 0 Likes
on
0 Posts
SOLVED! It seems I had failed to reset the cars ECU after fitting the new sensor, with the ECU reset the sensor started behaving normally and the car immediately went into closed loop with the fuel trim fluctuating up and down! I had previously done a reset with the old sensor and it didnt help so I'm satisfied that I did need to replace the sensor.
I should of said that the scanner does update about once a second, its only in datalog mode it seems to switch to 3 second update for some reason. Incidentally I dont get a CEL even if I unplug both O2 sensors (perhaps I would eventually?) maybe one of the differences between euro spec and US spec cars?
Anyway, thanks for you help.
I should of said that the scanner does update about once a second, its only in datalog mode it seems to switch to 3 second update for some reason. Incidentally I dont get a CEL even if I unplug both O2 sensors (perhaps I would eventually?) maybe one of the differences between euro spec and US spec cars?
Anyway, thanks for you help.
