My son's AX petrol has failed its MoT on emissions. It had a service including new plugs and air filter before the test and the garage suggest it might be the lambda sensor. Can I check what the resistances should be with a simple multimeter ? Anyone know what the values should be or where I could find out ?
Thank you.
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the garage suggest it might be the lambda sensor
some garages always seem to say that; do you have the actual readings from the emissions tester? This will at least show what the lambda was on the test.
I don't know about the AX, on some cars it is possible to check the heater coil for the lambda with a DMM. In terms of its correct operation I wouldn't personally think you can do much with a DMM, really need an oscilloscope. Hopefully an expert will be along shortly with some more specific advice.
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There are a few simple checks that can be done with a multimeter but not conclusive evidence that the sensor is faulty.Sensor disconnected check resistance across 2 white wires-approx 12 ohms(heater circuit).Engine running,sensor connected,battery volts acrosss 2 white wires.Also voltage across gray and black wires,engine hot,at approx 1200rpm should fluctuate between approx 0.2 -0.9 volts.hth
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Thanks for your replies.
Looks like we're getting somewhere.
The sensor (now I've found the blessed connector under the air filter) shows a dead short across the white wires and 13v across the supply so I'll replace the sensor and see how it goes. Didn't do a hot running check. The gray wire is badly frayed on the plastic clip as well.
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New lambda probe fitted, breather pipes cleaned and it's still an MoT failure. Lambda reads 1.02(pass), hydrocarbons 60(pass) but CO is 60 (or is it 0.6), 3 times what it should be. The garage suggest a new catalyser but I wanted to check on here if I was doing the right thing. Is the cat responsible for converting CO to CO2 ?
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A CO% reading of 0.6% is typical readings from a car with a failed cat. If you were to sample the gases prior to a working cat thats what you would see, the cat reduces the last bit down to near Zero.
May well have been a failed Lambda sensor that has seen it off but all may not be lost.
What sort of driving does the car get? Lots of short runs which dont allow the cat to fire up properly will gradually coat the catalyst with carbon. I have had spectacuklar results with a trade product called 'cataclean'. Available from factors for around £15 it has recovered several dead cats for me. Add to fuel (preferably when tank is near empty) and take for a good 10-15 mile spanking, not neccessarily high road speed but 50 in third say to get the engine and cat really hot. Its worth speculating a little before splashing out on a new cat. Dont be tempted to buy a cheap one, you gets what you pays for.
Andrew
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What sort of driving does the car get? Lots of short runs which dont allow the cat to fire up properly will gradually coat the catalyst with carbon...
Andrew,
Before the lad went to Uni, the car would have a brisk 15 mile hammer up the A1 to college and back 4 days a week. 2 or 3 times a week it would get a 20 mile run down the A1 to g/f's house and back (sometimes). The car has then stood for 3 months. I don't think pootling around has made the cat fail. Despite your kind recommendation, I think I'm going to put a new cat on.
50 in third ...
60 in third is the norm :-)
Thank you for your reply.
Edited by Hawkeye {P} on 18/12/2008 at 16:30
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