240 Constant o2 sensor issues

So we have another weird curveball of an issue to get some advice on.

So basically we're currently in the process of reffing my 1986 volvo 240 with a 1993 volvo 240 engine that we installed in place of my 1994 volvo 940 turbo engine...

Basically the car keeps failing due to a faulty oxygen sensor and throwing a check engine light 4 or 5 hours after we install a different sensor.

Keeping in mind we started with the 94 940 engine with 940t computers and had o2 sensor issues, then we got the current na engine from our 93 240 with the na 240 computers and have o2 sensor issues with the 93's sensor. But the catch is, that engine, computer, and sensor combo threw no codes in the 93 but throws codes repeatedly in my 86 no matter what known good sensor we put in 4 to 5 hours after putting them in....

We have checked wiring, everything seems perfectly fine.. And we have checked grounds and everything seems fine. Only odd thing is that when we use a multimeter between the ground and the sensor, the multimeter reads .5 volts and stays there as opposed to it bouncing between 0 and whatever.

We have no idea what's wrong and we've checked everything we can and we have a deadline, we're just taking any suggestions we can get at this point.

Typed this on my phone so feel free to ask questions if I left any important details out.

Thanks

Have you been cleaning the O2 connector with Electronic Contact Cleaner or anything else? If so, this can permanently damage the O2 sensor itself. The O2 sensor "breathes" or brings in a very tiny amount of reference air through the wiring harness. If Contact Cleaner is used, it wicks into the wires and the fumes then get drawn into the O2 sensor core causing permanent damage. The O2 sensor works fine after re-install, but then fails a couple days later.

Svenska_Monokrom said:

S

We have checked wiring, everything seems perfectly fine..

Click to expand...

not good enough.
replace, follow instruction to lettr

3 wires. 2 wires are heater pos and ground and third is signal. It makes it's own voltage for the signal.

Make sure a test lamp illuminates brightly between the heater positive and the ground wire with key on. If not, check fuse then recheck. If still no 12v suspect bad wiring and see if the 12v is missing from the pos, or check if the ground circuit is open. If the heater circuit is bad, the sensor will take too long to heat up and can cause that code.

Monitor the voltage of the sensor once it's hot. Snap throttle and it should go rich for a second .9v then go back to 0.1v momentarily. It should oscillate between 0.1 and 0.9v warm idle.

Please post back. I spent some time trying this out for you. I should be at the auto shop instead!

Svenska_Monokrom said:

... Only odd thing is that when we use a multimeter between the ground and the sensor, the multimeter reads .5 volts and stays there as opposed to it bouncing between 0 and whatever. ...

Click to expand...

If it is indeed steady and stable at half a volt, the circuit to the sensor is open, the sensor is dead cold, or mechanically broken inside. Reference contamination results in a sensor that still generates a voltage, however sometimes negative, and the result is you'd not have that 0.5V bias provided by the ECU. Remember, first edition Bentley has that error in it doing continuity checks toward the sensor. The two results "checking 24 and ground" are transposed.

bobxyz said:

Have you been cleaning the O2 connector with Electronic Contact Cleaner or anything else? If so, this can permanently damage the O2 sensor itself. The O2 sensor "breathes" or brings in a very tiny amount of reference air through the wiring harness. If Contact Cleaner is used, it wicks into the wires and the fumes then get drawn into the O2 sensor core causing permanent damage. The O2 sensor works fine after re-install, but then fails a couple days later.

Click to expand...

Sorry I forgot to mention that the sensors arent necessarily dying, we've tried 5 different sensors and all threw a code in my 1986 but wouldnt throw a code in any of the 4 other volvos we use. Weve kinda started assuming that its something that has to specifically do with my car but we dont know what else to check. edit:Until now at least.

As an addition to the weirdness, my car takes a few hours to know if the oxygen sensor is even unplugged but in my friends 242 the code would typically pop up within a 3 miles drive.

Done the tests?

ZVOLV said:

Done the tests?

Click to expand...

Haven't gotten to it yet, we're gonna try it out today or tommorow and post what happens.

edit: were making a ghetto test light to use now

You can use a meter too, but test lamp is better. 12v can pass thru 1 strand of wire, but won't operate the circuit.

I would take the 5 seconds to confirm less than a couple of ohms on the ground circuit. If all checks out... I would then continue to check continuity between the signal wire terminal (o2 sensor side) and the corresponding ECU terminal. Back probe only!!! Wiggle the wiring and make sure the resistance stays no more than a couple ohms.

If all the wiring checks out, and the sensor is good, I would fire an ECU at the problem. Maybe even run new wiring too. Check for bent/loose terminals too. I do drag tests to test terminals.

Any update? I was thinking the sensor didn't have 450mv bias voltage from the computer on these old OBD systems. Anyways, you can unplug the sensor and see if it's coming from the computer.

You could unplug the sensor and monitor the voltage of an unplugged sensor. You can do snap throttle, introduce air leaks, or add alternate fuel and monitor if the sensor is alive or dead.

You can even remove it and use burning propane. Here is a video proving that the o2 sensor makes it's own voltage using the burning propane method: https://youtu.be/NKsS6AELPuM

It sounds like you have a wiring/circuit issue and your sensor is fine.

It turns out this problem was because of a mistake I made when I originally swapped this car to LH2.4. Apparently during the installation I never connected the power wire for the O2 sensor heater which explains why the issue persisted between engine swaps. On the other two LH swaps I did I swapped the complete chassis harness along with the conversion so I never knew the heater circuit passed through the 9 pin harness connector. Today I checked the heater circuit and there was zero voltage so I followed the wire back through the car and it was just sitting above the fuse box disconnected. Thanks everyone for your help solving this I wouldn't have thought to check the heater circuit otherwise. I never knew the heater circuit was so important for the O2 sensor to function at all I just assumed it was there for faster warmup.

That also made it quite apparent that basically all of the like 6 o2 sensors we tried werent actualy not bad. So we collected alot for pretty much no reason...

Luckily we never attempted moving on to trying different computers. That would have gotten expensive quick.

But anyways guys, thanks for the assistance.

Heater is very important. On older cars there isn't a heater circuit code. If the heater circuit is bad, the sensor won't warm up fast enough and the computer sees that and then will trigger a code

On modern cars, the computer monitors the current/voltage going thru the heater and will toss a code very quickly if there's an open circuit.

Replacing parts based on codes alone is a mistake. You always need to confirm the circuit is good first.

Glad you got it and that I could help. Thanks for posting back. When I can help somebody like this it feels good and that keeps me coming back.