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[chrisr]

I had the radiator replaced on my 2002 petrol about two weeks ago and all has been well since. On Friday  I drove about 85 miles to my daughter's with no problem. Today I headed home and after about 5 miles the check engine light came on. Then the check coolant warning came up in the car. I checked and it was ok. I got home and the coolant was still ok but the check coolant level warning is still on. I put my OBD2 scanher on and it said the fault was P0420 but it wouldnt clear. I'm thinking I need to change the after cat  O2 sensor again as this solved the check engine light a few years ago but whats with the coolant level warning? Are they separate problems or connected? The car is still running fine.

[Doctor Gollum]

Separate problems. Did you remove the reservoir during the radiator change?

If yes, check the sensor plug.
If no, check that the float is still present and bobbing.

There should be a plastic washer over the top of the float. If this is missing it can potentially obstruct coolant flow.

Only guaranteed solution is to replace the tank.

[TheBoy]

If the float is OK, short out the connector plug with a paperclip and see if the message disappears. If it does, that's also a new coolant bottle.

The P0420 is indeed a common issue where the precats are tired.  Move the sensors rather than replace the cats with an even crappier pattern version

[chrisr]

New coolant tank and after cat lambda sensor on the way. Lets hope its not the pre cat one! Thanks guys.