I have a 2012 1.2htp.

When i bought it it had a check engine light and i scanned the code it had a code for the bank 1 lambda sensor. I also noticed that it had a non genuine catalytic converter in it. I then failed on emissions (see the readings beside the pink highlighter)

After i failed emissions i scanned codes again and catalytic converter inefficiency came up.

I then bought a genuine second hand catalytic converter and bank 1 O2 sensor, i ran this for about 150 miles and the codes never came back.

So I then put it through another NCT (MOT)

As you can see from the picture with no highlighter, the Lambda has now got even worse, the CO has increased, and the HC has improved slightly.

Where do I go from here. I obviously have some sort of rich fueling problem but with no fault codes I'm not really sure where to look?!

Any help would be greatly appreciated.

Edited 11 June, 202511 Jun by Gondard
Changed picture

Which oxygen sensor did the fault code indicate, and which did you change? Both are bank 1, there's bank 1 sensor 1 and bank 1 sensor 2?

Top of cat or after?

Do you have a record of that fault code somewhere?

Have the spark plugs been looked at or changed since purchase?

Does the car have a coolant temperature gauge?

Edited 11 June, 202511 Jun by Breezy_Pete

Hi Pete, I changed the sensor that's on top of the Cat. I meant bank 1 sensor 1, which is the one i changed.

I've recently done a full service with oil and plugs last week.

I don't have a coolant gauge no..

I no longer have the fault code available it's gone with the installation of the new cat

23 minutes ago, Breezy_Pete said:

Which oxygen sensor did the fault code indicate, and which did you change? Both are bank 1, there's bank 1 sensor 1 and bank 1 sensor 2?

Top of cat or after?

Do you have a record of that fault code somewhere?

Have the spark plugs been looked at or changed since purchase?

Does the car have a coolant temperature gauge?

Do you have access to a scan tool that can read coolant temperature in real time?

4 minutes ago, Breezy_Pete said:

Do you have access to a scan tool that can read coolant temperature in real time?

Yes I've got vcds

Oh, that's good.

Look to see what it reads after the engine has been running for 5 and 10 minutes, just in case the temperature sensor is giving implausible readings.

Also have a look at what bank 1 sensor 2 is doing, measuring block 36 or somewhere round there.

6 hours ago, Breezy_Pete said:

Oh, that's good.

Look to see what it reads after the engine has been running for 5 and 10 minutes, just in case the temperature sensor is giving implausible readings.

Also have a look at what bank 1 sensor 2 is doing, measuring block 36 or somewhere round there.

Hi Pete, was reading the sensors and it seems that both sensors are working correctly, same with the coolant temp sensor, you can see from my live data that my lambda is rich at idle and at constant revs.

Not sure if it helps but my sons 1.2 htp had same problems getting through last MOT re emissions. MOT guy noticed a very slight leak in centre exhaust joint.(my fault as I hadn't tightned it properly after fitting a new front section. He sealed it and tightened it and emissions were then good. It was a tiny leak but obviously enough to affect emissions. He reckond that the engine may be sensitive to the wrong back pressure but couldn't understand why such a small leak would make a difference.

Alasdair