So, a while back I posted a query here about the O2 sensors, as I was getting error codes for all 4 in my 2005 XJ8L. It seemed very strange to me that all 4 would fail at the same time, but no one posted any ideas, so off to the dealer it went.

Of course, they said: all 4 sensors are dead/burned out, need to be replaced. After I pointed out how weird that was, they cleared the codes, took the car out for a drive, and the codes re-appeared, so I okayed the work. When it was time for the car to be picked up, the service mgr said that they had found a relay that had shorted or failed, and that had caused the problem, burning out the sensors.

Now, I am wondering if in fact the problem was with only the relay, but not the sensors.

I have the sensors in my possession; is there any way to test the functionality of the sensors out of the car? I would really like to make sure the sensors are in fact non-functional, as I will be very upset if the dealer replaced something that didn't require replacement, particularly given my concern about the low likelihood of simultaneous sensor failure.

Thanks for any help you can provide.

Lance

 

Because dealerships are driven to figure problems out quickly, they may not have actually verified the dysfunction of the oxygen sensors. Using SDD, it is possible to monitor the output of the oxygen sensors.

I am guessing the sensors are the 4 wire type, a combination of heater and sensor. I am also guessing that the sensors are binary and not a wide-band type.

The biggest problem of testing the sensors outside of the car is having a hot gaseous hydro-carbon stream available.

So, the following is a suggestion of how you could test. I take zero responsibility with this.

Tools needed:
Car or motorcycle type battery.
Propane torch.
Jumper clips.
Sensitive low impedance multi-meter or oscilloscope.
Gloves
Eye protection
Fire extinguisher


1. Collect everything together outdoors where there is plentiful fresh air and very little chance of ignition. (this is propane we are working with here)
2. Don the gloves and eye protection, have the fire extinguisher ready.
3. Attach jumpers to the four terminals of the oxygen sensor.
4. Attach the jumpers from the heater part of the oxygen to the multimeter. (oscilloscope cannot be used for this test)
5. Set the multimeter to read Ohms (resistance, the greek letter omega).
6. The multimeter should display a value between 0 and 100 ohms (just bear with me techs)
7. If the value is over 100 ohms, for example 10k ohms, or 100M ohms, or infinity, the heater in the sensor is broken.
8. If the reading is good, go on to step 9.
9. Attach the jumpers from the sensor part of the sensor to the multimeter or oscilloscope.
10. Set the multimeter or oscilloscope to display 0 to 1 volt DC.
11. Attach the jumpers from the heater part of the sensor to the battery.
12. Turn on the propane torch, but do not ignite it.
13. While watching the multimeter or oscilloscope, put the torch tip to the sensor cage of the sensor. Then move it away.
14. Repeat.
15. You are looking for a 0 volts to 1 volt change (or 0 volts to -1 volt change) when you put the stream of hydro-carbons to the sensor cage. And a 1 volt to 0 volts change (or -1 volts to 0 volts change) when you move the stream away.
16. If you get 0.1 volts to 0.8 volts (or -0.1 volts to -0.8 volts) the sensor is good.
17. If you get 0 volts to 0.5 volts (or 0 volts to -0.5 volts) the sensor is bad.
18. If the reading stays at 0, or if you get no change, then either the sensor is bad, or my methodology is bad.

 

I have looked at the wiring diagram and there is a relay just for the O2 sensors and a fuse on the supply to the heaters.

If the relay failed why didn't the fuse blow and protect the sensor heaters?

The actual sensors themselves are connected to the engine control module and it cannot have been a problem on that side because you have not had a problem with the ECM, besides the relay which has supposedly failed only supplies the heaters.

If the heaters have failed (burnt out) they will be open circuit so you just need a continuity check across the heater connections.

IMO it is a scam!

Some dealers will lie, cheat and steal.

If this post is censored I will be very annoyed!

 

We call them *******s.

 

I concur...I doubt the sensors were all bad. You can check the sensor heat by checking the ohms between the two white wires on each sensor. If there is continuity, the heaters have to be intact. I guess it could be possible that the heaters all fried, but, more likely they didn't recieve power, which triggered the "slow to heat" code.

Now, on the bright side...at the 8 year point you were likely going to have to replace all 4 sensors in the next 2-3 years anyway, so you just got it all out of the way at once!?!

John