P0421 mk3

[ARHarh]

Can someone help confirm my diagnosis?

I have a 2007 2.0 sport. It has started flashing up a p0421 code. So using the toque app I have investigated the lambda sensors. It appears that sensor 1 (pre cat) is not working as there is no voltage showing on the graph. Sensor 2 is responding ok. So if sensor 1 is not showing fluctuation in voltage is it broken? If it is can i replace it with a generic sensor?

Thanks for any help

[SmilerFTM]

The O2 sensors are both in the midpipe, have you had a new manifold without a cat built in fitted?

[ARHarh]

No car is standard. I know where the sensors are.

[Bertrum]

Check the wires back to the Plug, they can touch the exhaust if the clip is broken that secures the plug and melt the insulation.

The O2 sensors are expensive from MX5 parts.

One of them requires a cut out socket to get off you can get to them from under the car when you take the under tray off. You won't be able to reach from above.

If the O2 is not reading, it should flip back and forth it is broken however also check your coolant sensor to make sure that is working correctly as if it fails it will cause the O2 to not be used and the same fault.

Edited by Bertrum on Thursday 23 February 13:16

[ARHarh]

Thanks

[nickthecynic]

SmilerFTM said:

The O2 sensors are both in the midpipe, have you had a new manifold without a cat built in fitted?

Smiler - if you're still on here, I'm getting P0421, have changed the sensors and fitted a new manifold without the pre-cat but still getting the EML every 50-70 miles. Is there something I need to know?

[ARHarh]

nickthecynic said:

SmilerFTM said:

The O2 sensors are both in the midpipe, have you had a new manifold without a cat built in fitted?

Smiler - if you're still on here, I'm getting P0421, have changed the sensors and fitted a new manifold without the pre-cat but still getting the EML every 50-70 miles. Is there something I need to know?

This was while ago. But It ended up being the manifold pre cat crumbling. A big lump of pre cat was blocking the airflow to the o2 sensor in the bottom of the manifold, this then blocked the main cat. New (second hand) manifold and new cat solved the problem. Been fine ever since and still in my ownership.