02 Sensor Issues - Update - Problem Solved (I hope)

Hi Guys.

Previously I posted on my cars 02 sensor issue with my Mines ECU installed. I was experiencing lean condition on idle and the ECU had fault code 21, main O2 sensor fault.

I assumed the O2 sensor faulty and replaced with a new Bosch sensor. Same condition, additional problem was running lean whilst driving, at low throttle opening.

I replaced the ECU with my original one, same lean condition whilst driving.

Now I have gone back to my old OEM sensor - no lean condition whilst driving.

The old sensor voltage switches between 0.7V and 0.4V, but occasionally it drops to 0.02V.

What exactly does this mean? Is the old sensor faulty as well or is it a wiring fault?

Can a diagnostic check with the Toyota Intelligent tester tell me if there is a wiring fault?

Any help is appreciated.

Anyone with a digital oscilloscope can tell you if the O2 sensor(s) are OK. 5 minute job. I test all cars in for a service nowadays. Normal voltage swing on a MKIV is 0.2 to 0.9 or so volts cycling about every 0.6 or so seconds at fast idle.

Here's a screen shot of a healthy front sensor on a UK car, Jap spec will look the same. This test was done by snapping the throttle open off idle to check the sensor response and decay time.

Chris,

I have no god parents, im wondering if you want the job? This would inc me picking your brains for hours on end about pointless stuff?

What's the hourly rate for God parents these days?

I would pay you by annoying the crap out of you by trying to talk about stuff i have no idea about?

Back on topic, Are 02 sensors still working in 15 year old cars? I would be interested to see what % of supras are running broken ones.

There are plenty of MKIV's running around with dead O2 sensors, not being fully OBDII compliant they do not necessarily flag a code, and the only hints are poor light throttle performance and excessive fuel consumption. If someone buys one with a dead or tired O2 they may well not realize that all is not well with the car.

Would the fuel/air ratio still be 14.6 with a dud o2 sensor?

Possibly close at idle, but not at light throttle cruise if it was totally dead.

I have a spare O2 sensor if needed mate.

Thanks for the replies mates..

I dont have access to a digital oscilloscope, need to check if the garage I use has one.

Based on the voltmeter reading of 0.02 - 0.7V, I reckon there is an issue. I will check out the wiring on the ECU, if no issues then it has to be a duff sensor..

not being fully OBDII compliant they do not necessarily flag a code, and the only hints are poor light throttle performance

Thats the issue I had with the new Bosch sensor. It was definitely dead as there were times when it was running lean whilst driving on light throttle and it still did not flag a code.

The weird part is that the Mines ECU flags a code almost immediately, car does not even have to be running.

Update...

Installed the Mines ECU back into the car..

Checked for error codes with the key in "ON" position. No code.

Switched it off and started the car, 02 sensor error returned.

Left the Mines ECU in and drove the car, idle AFR was between 15.4 and 14.7, same with light throttle cruise.

The sensor seems to be funtioning ok, it constantly swings bet 15.4 and 14.7. The voltage is still between 0.9V and 0.2V, but one point is going down to 0.09V.

I have a brand new Denso sensor on order, if I still get odd voltage with that then I reckon the ECU or ECU plug is the culprit.

Is my thought process correct?

Well the 0.2 to 0.9 would be correct response voltage for a good sensor, and so is the swing between 14.7 and 15.4 on idle and light load/cruse, what are your AFRs on boost?

Well the 0.2 to 0.9 would be correct response voltage for a good sensor, and so is the swing between 14.7 and 15.4 on idle and light load/cruse, what are your AFRs on boost?

AFR on WOT is in the 10's... with around 50% throttle I see 12's...

There is that one point were the sensor voltage drops to 0.09V...

It pops up every few cycles..

Ok, got home from a spirited drive from work.. AFR was fine, except when I parked at home..

It started creeping up on the lean scale...

O2 sensor was reading a rock solid 0.02V..

I reckon the sensor is stuffed?

Its an odd one, failed/failing O2 sensors usually exhibit a slower response rate to switching of fueling by the ecu, or just show as fixed high or low, normally low,

as yours is, but once there, they usually stay low, i am starting to wonder if the is a heater circuit problem, but once the sensor is hot, that shouldn't be a problem either.

The sensor was new, but I had an issue with the boost pressure wire pulling out at the ECU, car went mega rich, I dont know if that could have messed up the sensor..

Since then I had issues with the car running lean on idle.

Then I fitted the new Bosch, had the issue with the car running lean on idle and part throttle.. (I tried both ECU's)

So I went back to the old sensor..

It may be wiring but I am not sure where to start with it or what to check..

i am starting to wonder if the is a heater circuit problem, but once the sensor is hot, that shouldn't be a problem either.

After reinstalling the Mines ECU, I did the diagnostic check with the key in ON position, no code, switched it off and then started the car and it picked up the code immediately..

If I cleared the code after that, did the diagnostic check with key in ON, ECU picked up fault after 2 flashes..

Does the heater circuit function before the car starts?

Does the heater circuit function before the car starts?

Yes its intended to bring the O2 sensor up to working temp as quick as possible.

Yes its intended to bring the O2 sensor up to working temp as quick as possible.

Then it may be a heater circuit fault if the ECU picks it up before the car is running...

I will try to refurbish the ECU plug, then fit the new sensor..

Used the car to work..

Perfect AFR's..

O2 sensor voltage was back at 0.7V to 0.2V with the occasional dip to 0.09V..

So today decided to refurbish the 02 sensor and heater wires on the ECU plug. Plan was to take the harness out of the cabin to make it easier to work.

I dont have a garage here so I was out in the freezing 2 deg afternoon, plus the wind kick up making it impossible to work.

Long story short, all I did was jiggle the heater circuit pin around abit, reconnected it and its not picking up any error.

I think the plug was not making contact on the Mines ECU.

Lets hope its solved...