I have a 1993 WRX with a de-cat 3"-bore centre pipe (but still got the OEM catted downpipe which, if original, has covered over 92,000 miles).

Here are the results of my MOT emissions, which failed on the 2nd fast idle test...

Natural Idle - CO 0.38%... PASS

Fast idle #1 - CO 4.01% / HC 124ppm / Lambda 0.914... OVERALL FAIL

Fast idle #2 - CO 1.88% / HC 78ppm / Lambda 0.97... OVERALL FAIL

Do the above results indicate that the lambda sensor is faulty, rather than the CAT. The reason why I think this, is because the hydrocarbons are low in both fast idle tests (especially so in test #2)? Or would a knackered lambda never be able to make the effective test zone of Lambda 0.97 to 1.03, anyhow?

Any advice would be most helpful, as I don't want to unnecessarily spend out on parts speculatively, in a vain hope of sorting the emissions by trial and error.

Thanks, John

EDIT: At least the motor is running 'safely' rich!

The cat could be faulty as well, but the lambda readings are wrong indicating a rich mixture. Check the lambda voltage first! The signal wire is the white one at the car end of the multi plug. You should see it cycling, but it probably won't be. If the voltage is steady high, at around .9 then suspect something else causing the rich mixture. If it's sat around .3 to .5 of a volt and not moving, change the lambda sensor. You'll have to sort that first, then go back for a re test. Chances are it'll pass!
Best of luck.

It's probably the lambda, it will most likely be sat at 0 volts, and causing the ECU to add fuel.

Thanks for the reply Alan.


If the voltage turns out to be stuck at 0.9v, I thought that would be the reason for a rich mixture in the first place(?) - a bit confused here. If the Lambda's voltage oscillates up and down between 0.1v to 0.9v, then does that mean it's working correctly - and, therefore, I should look elsewhere for the problem, yes?

Is it possible for you to briefly explain further, so that I understand, more correctly, the way the lambda works and sends/receives info to/from the ECU? Sorry to impose.

Regards, John.

stop asking questions, start measuring!

Sorry.

Zen Performance, do you stock decent quality lambda sensors (e.g. OEM Subaru ones) for my model car, and if so how much supplied and fitted? If so, please PM me (if you're not allowed to discuss prices on the forum)...

Thank you, John

Zen talk sense. Measure first, then tell us about it!

For those interested
Low signal voltage means the AFR is lean (.1)
High signal voltage means the AFR is rich (.9)
Medium signal voltage means the lambda isn't working. (.5)
Cycling between the two means AFR ok!
If it fails the MOT on high CO when the lambda is cycling, suspect the cat.
The above only applies to early Zirconia sensors, and is not intended as the final word, just a simple guide to sussing out the problem.

Cheers Alan. Understood!

Without wishing to sound thick (I'm no electrician or physicist)...

I've just bought a cheap multi-meter and when I go to test the sensor, to get a reading, am I supposed to...


...unscrew the sensor's head from the exhaust manifold...

...run the engine up to temp...

...touch the positive probe onto the end of the sensor head...

...whilst touching the negative probe to earth on a bare metal area of the car chassis , to create a circuit?


Am I barking up the right tree, anyone?

Thanks, John

use a pin pushed up the back of the connector to the lambda sensor to measure the voltage on the white wire, assuming you have red/black/white as per standard, with the -ve lead from the meter on a good engine ground (like inlet manifold).

Cheers Paul. I was aware (in the end ) that's how to do it - thanks for replying!

At last, finally got round to testing - here are the results...

- On warm up (straight from a stone cold engine)... reading stuck at 0.2V

- On a thoroughly warmed up engine (after about 40-60mins of driving, and engine idling whilst measuring)... reading stuck at 0.3 to 0.4V (but mainly at 0.4).

These results appear to be exactly as Alan Jeffery described (see below)...

...So, presumably, indeed a duff Lambda, yes?...

Cheers, John.

yes duff lambda.

Thanks Paul, for the confirmation. And thanks to everyone else, too

AAAARGH! Don't believe it. Checked again this morning and got different results!...

With engine up to temp and idling... reading definitely appears to now be cycling up and down, between 0.1/0.2/0.3 and 0.5/0.6V - but, on average, cycling between 0.2 and 0.5V!!!

What's the story with that - is it still a dud Lambda, or is it actually working correctly and within spec.?!...

Cheers, John.

lamdas dead! should switch between 0.1 and 0.9. but the sensor could have killed your cat, change the sensor and check co.

Yeah, I thought it was still dead, but just wanted to definitely be sure. Thanks