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[terry paget]

Thanks.
 When I did my pre-MOT check there was a P170 code showing, but I cancelled it and it has not returned.
I thought the post cat-sensor did most of the work, and the pre-cat sensor only functioned during cold start times. Is this so?
My MaxiScan MS309 does not appear to read live data, though  manual does have a section on Retrieving I/M readiness status. Would a TechII give more information?

No lamda is used for warm up (as lamda needs to be hot to work), front one controls fuelling, hence you should see it constantly flick rich/lean at about 1hz.

Rear lamda only fitted to OBDII cars, and is used purely to monitor cat efficiency.

Thanks. It sounds like I have changed the wrong lambda sensor. Should I now change the 1st sensor? What instrument would allow me to see fuelling flicking  rich/lean?

[TheBoy]

Any code reader with live data functionality...

[terry paget]

Thanks. I shall change the front sensor and resubmit for test.

[aaronjb]

Someone correct me if I'm wrong, but if the lambda reading at the tailpipe was true, wouldn't the car be seriously struggling to idle? With an AFR of 16.5:1 to 22:2 would it not be sounding very unhealthy, hunting, rattling and generally sounding like it's in it's death throes..

.. that's certainly how the MR2 sounded when I got the idle tune a tad wrong and ended up with an 18:1 AFR, anyway!

My money is, as others have suggested, on an air leak allowing air into the exhaust after the first O2 sensor.. and I think you're potentially throwing good money after bad by changing the sensor without seeing live data first..

[terry paget]

Someone correct me if I'm wrong, but if the lambda reading at the tailpipe was true, wouldn't the car be seriously struggling to idle? With an AFR of 16.5:1 to 22:2 would it not be sounding very unhealthy, hunting, rattling and generally sounding like it's in it's death throes..

.. that's certainly how the MR2 sounded when I got the idle tune a tad wrong and ended up with an 18:1 AFR, anyway!

My money is, as others have suggested, on an air leak allowing air into the exhaust after the first O2 sensor.. and I think you're potentially throwing good money after bad by changing the sensor without seeing live data first..

Thanks for your input, aaronbj. Trouble is I cannot find an air leak into the exhaust after the first O2 sensor. Originally I thought I had one at the flexi, but this morning I found it was not a leak, just a bit of damp gauze. It was silent, too. The sensors all look the same, so I have spares, including the one I took off the post cat hole so is known good, so it will not cost me money.

The car idles sweet as a nut, pulls well and behaves impeccably. It's a mystery to me.

If it fails MOT again on lambda, I shall ask the tester to plug in his flashy meter, and check live data and see what he can find.

[terry paget]

MOT passed today after changing first lambda sensor for spare. Exercise has taught me a lot, advise from OOF has been invaluable, thanks to all.