Something not right here .The upsteam sensor (top sensor should be fluctuating between around 0.2-0.8 volts and the downstream (lower) sensor should be static at around 0.4 -0.6 volts.

I've seen 0.2-0.8v on the upstream sensor at 3000rpm, but not at idle. Will recheck tomorrow. Ta for the pointer as to what I should expect.

Thanks for the replies. Think I may have fixed it, though I'll only know for sure when I go for retest.

Doc - On looking at ODBWiz graph showing O2 sensor voltage fluctuations, with a properly warm engine at 3000rpm the upstream O2 sensor was reading 0.2v - 08.v, while the downstream was reading 0.7v -0.8v. The downstream sensor voltage is higher than what you quoted, but I'm not worried there, as the car DID pass the fast idle test at this RPM.

But at idle (640rpm) I was getting steady 0.8v and 0.9v on the upstream and downstream sensors respectively. So fuelling as if the engine was cold?

Then I tried swapping the throttle body with one from the scrappy. Looks like Focussed was onto something, for after the swap I was getting the expected voltage fluctuations at idle (640rpm) - 0.2v - 0.8v on the upstream sensor, approx 0.8v steady on the downstream. A revving problem was fixed too.

By swapping the throttle body,you have also changed the maf sensor .this would account for your emissions and revving problem.Early micro k11 models run a completely different injection system with a different throttle body,that are a common issue.

Aye, I know the MAF sensors on the later K11's are meant to be better, but I have seen forum posts with people having grief with these too.

Think I will go for the retest. It'll be a while before I get a chance to take it in, but I'll report back on the result

I wouldn't get too excited. The output voltage from the O2 sensor after the catalyst is for reference only. It merely tells the ECM that the catalyst has done its job, or not as the case may be. The ECM does not respond to to the information received from it like it does from the pre-cat sensor.

The interesting bit wasn't what the post-cat sensor was doing, it was the voltage fluctuations on the upstream (pre-cat) sensor.