Is it Possible to Check O2 Sensors?

[WIL35]

The orange 'Engine Malfunction' light has lit up on our petrol 2003 Toyota Avensis Verso. This happened a while back and I fixed it by replacing one of the post catalyst O2 sensors. The car has four sensors, two before and two after the cat, is it possible to check each one's operation by using a multimeter with them in situ? They are the four wire type, so I am assuming two wires heat up the sensor and the other two send a signal.

I know I could take it to the Toyota dealer for diagnostics, but that is a bit inconvenient at the moment.

Thanks

[stevieturbo]

Depends if they are wideband sensors or not. Which is a good chance they are.

Easiest way is with proper diagnostic equipment.

[buggalugs]

You could be lucky and it will be an obvious fault. I replaced the 4 wire sensor on my car a few weeks back and the impedence across the sensor wires was a meg-ohm or so on the knackered one, and infinity on the new one.

If you've got a high impedence volt meter you could measure the signal output while it's running too, if three are moving around .45v and one is less than that then that one would be a fair bet.

You can also check the impedence on the heater circuit.

This is assuming that the light isn't on for a different reason this time!! Older toyotas could be made to flash the light at you to tell you what was up, not sure about newer ones.

HTH

Neil

[ELAN+2]

Both pre cat Lambda's should be switching between .1volts (lean) and .9volts(rich) with the engine hot and idling, both post cat should be around .45v (cat working) and stable. the best way to check is with an oscilloscope are a very good graphing multimeter, If a sensor isn't switching either its knackered or there is another issue, i.e. an air leak or dribbling injector/burning oil. If the sensor is giving a high voltage(rich), then create an air leak after the mass airflow meter, (force it lean the sensor should react instantly by dropping to .1 volts), if its showing a low voltage(lean) block/clamp the fuel return from the rail and the sensor should register a rich mixture/high voltage. If the sensor doesnt react, its knackered. service life for these is around 60-80K miles dependant on application. The four wires are as follows: 2 (Usually white) are the internal heater circuit for the sensor, the ground is usually grey and the signal wire black, use a paper clip or a piece of welding wire to "backprobe" the signal wire at the car loom end of the connector. Do not resistance test whilst connected, just read the voltage.

enjoy

Mark

[WIL35]

Thanks for that, I'll print it off for future reference because the light has now gone off!

I just ordered a new sensor and my wife says the light isn't on anymore. I thought malfunction lights could only be switched off with the required code reading equipment or by disconnecting the battery or relevant fuse.