Audi A1 High CO

[CuriousSpaghetti]

My 2015 Audi A1 recently failed its MOT with the following emissions:

Fast Idle:
CO: 4.671 (0.2 max) FAIL
HC: 123 (200 max) PASS
Lambda 0.870 (0.970-1.030) FAIL

2nd Fast Idle:
CO: 2.768 (0.2 max) FAIL
HC: 44 (200 max) PASS
Lambda 0.925 (0.970-1.030) FAIL

Natural Idle:
CO: 1.986 (0.3 max) FAIL

But then the next day, following the installation of a new lambda sensor, it passed with 0.000 CO on both idle tests and 1.000 on the lambda (perfect results). Is it likely that a 4.671 CO reading could truly be caused by a faulty lambda sensor, or was it potentially measured wrong initially? Also, is it normal to get a 0.000 result, as that seems too low to be true.

[stevieturbo]

It's clear from the first result it is far too rich.

And yes, a malfunctioning lambda sensor can cause incorrect fueling.

And an absolute 0 reading is very unlikely, but as the cats job is to convert CO to CO2.....in an ideal world potentially it could be zero.

[CuriousSpaghetti]

Excessively so. I’m curious as I thought I’d have noticed an issue with the car performance or a warning light if it was that rich. I was just wondering if it seemed more likely that the garage have deliberately exaggerated the readings somehow to make me purchase a new sensor. The apparent readings of 0.000 on the re-test seemed suspicious too.

[stevieturbo]

Unless they put the probe un another cars exhausts, there would be no easy way for them to fake the result, or started unplugging random sensors to mess up normal operation on the car.

Lambda sensors don't last forever, they need replaced...that's life. And there are literally dozens of issues on cars that won't offer fault warning lights.

[Chris32345]

If it's an older model emissions tester I'd say its definitely possible it could be a zero reading older systems are likely to be less sensitive especially as they age

[stevieturbo]

Chris32345 said:

If it's an older model emissions tester I'd say its definitely possible it could be a zero reading older systems are likely to be less sensitive especially as they age

And are all supposed to be regularly serviced and calibrated.

[Chris32345]

stevieturbo said:

And are all supposed to be regularly serviced and calibrated.

Still dosnt change the fact that older systems will still likely be less sensitive
Maybe not hugely but easily could be the difference between reading 4ppm co and 0ppm

[Zener]

stevieturbo said:

It's clear from the first result it is far too rich.

And yes, a malfunctioning lambda sensor can cause incorrect fueling.

And an absolute 0 reading is very unlikely, but as the cats job is to convert CO to CO2.....in an ideal world potentially it could be zero.

This ^ and an 02 sensor failure wont always put the instruments MIL light on even though its a current fault code that inc the 02's heater function too , its fixed be content thats all it was it dont get much easier/straight forward of a repair than that , its considered a consumable item after all

[CuriousSpaghetti]

Thanks everyone! Was very happy it fixed it as was worried I needed a new catalytic converter already.

[Zener]

CuriousSpaghetti said:

Thanks everyone! Was very happy it fixed it as was worried I needed a new catalytic converter already.

A failed catalyst will never give/cause high CO readings like you posted however