Hi guys I am at a total loss what’s happening with my 2007 R56 1.4 petrol Mini One.

No engine light is showing, or codes recorded, yet it fails each time on the 2500 rpm CO level which is 0.8% at the test station. It had a new Cat 2 years ago, passed with no problem last year, new air filter and oil done a few weeks ago, tried Cataclean but no improvement. I purchase a Gunson Gastester which shows 0.8% at idle, and 0.4% at 2500 rpm below are the photos of the results using a OBD2 reader at idle, can anyone give me advice on what else to check, or this immaculate mini will be heading for the breakers ☹

 

Most likely just an o2 sensor problem, resulting in running rich. Might have been your real problem 2 years ago, too. When the sensors age they can read wrongly, in either the too rich or too lean direction.

Failing that, older injectors can develop really bad spray due to clogging, which causes high CO.

I'd try a compression tester, to determine that the engine is generally healthy. Then fix the issue, if so. Not quite ready for the scrapyard just yet....

 

I hope not, from what i have researched the post cat O2 sensor should read around 450mv mine reads 725 mv checked the compression all within 5lbs of each other

 

You want to read the pre-cat O2 sensor, the one right at the top of the exhaust manifold. On your reader it would be B1-S1. The post cat O2 sensor (B1-S2) only monitors the catalyst efficiency. Also, just because it reads within normal range does not mean it is giving the correct reading to the ECU.

 

When the O2 sensor is aged and inaccurate, but not completely failed, the engine management will still force the sensor to the right reading. That's what closed-loop means. It adjusts fueling to make the sensor read what it should.

So what you get is everything is happy, the engine management is dumbly doing what it should, no check engine light, just high CO and subtly worse fuel economy.

 

Hi pre cat O2 reads 1.39v do not know if thats good or bad

 

Thats interesting I may just replace the cat and both sensors, its just such a pain to put the front end into service mode :-( was hoping it was a bad sensor

 

Update...I changed the Post cat O2 sensor, before at idle and 2500 rpm it hovered around 0.725v, now with the new sensor it varies under throttle, and at 2500 rpm hovers around 0.500v I believe it should read 0.450v so I am going to take it for another test to see if it passes

 

Well, I'd have changed the pre-cat one first, as that's the important one that actually governs the fuel/air mix. But it's your car....and your money.

 

It passed !!!!!!!!!! the problem was due to the post cat O2 sensors readings, thanks for everyone's suggestions

 

Excellent!