J-Spec TT 1993 Failed MOT again

OK, have refitted the first cat but it is still failing. My lambda readings are worse as are the co levels after fitting a cat back in for the dreaded MOT which it failed again.

The car has been off the road for a year getting repaired so this won't help. I've seen Nics picture of the lambda sensor in another thread but that isn't as long as the sensor that fits into mine after the cat. Could this be at fault and what is this second sensor?

Anyone any ideas please.

Thanks in advance

Hi Simon

The second sensor is most likely the CAT temperature sensor, this will not effect the emissions, it just senses if the cat is overheating, normally these are tied up out of the way when the CAT is removed.

The O2 sensor is the first one fitted to the exhaust manifold, if it hasn't been replaced for a couple of years, then this is probably the reason for the emissions failure. Removing and cleaning with carb cleaner may help you get through the MOT, but ideally you should replace with a new one. Also try resetting the ECU.

HTH

Simon,

Don't know if its applicable to the soop, but there's a number of sensors that can affect fuelling at idle.

First there's the air temperature sensor, measures the incoming air temp. If its reading wrong (broken, unplugged or fallen out) it'll affect fuelling.

Uk Spec cars have a MAF meter. If this is dirty/oily (hot wire type) this can read wrong, thinking it has there's more air than there actually is.

The coolant temperature sensor increases fuelling for a cold engine. If its knackered & reading a low temp all the time, then CO goes up.

O2 (lambda sensor). Gets old & stops working. It should swing back & forth between 0.3 and 0.9 volts at hot idle as this is how the engine averages the fuelling. You can probably only see this on a scope. Don't know what it is on a DVM (volt meter) cos I haven't tried it.

The 02 sensor can be tested at home. Pull it out, stick a DVM on it and point a propane blowtorch at its tip. This will heat it up & burn off the crap in it. Should read 0.9v between the black wire (signal) and the grey (ground) when warmed up nicely.

Best bet is to replace it if its 60K miles old or more, or if you don't know when it was changed.

Hope this helps.

OK thanks guys will try the lambda sensor. Yes, NIC, I remember now, just a temp probe, thanks for jolting the memory.