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Legacy b spec fault code p0420

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Hi my husbands legacy has a intermittent engine management light on and is showing a p0420 fault code !! Local dealer said it was a AS sensor that needed cleaning , and did so at a cost of over £400 , we were told the sensor may need replacement , and if engine management light came back on to return the car for replacement.

On trying to book the car in for the sensor replacement I was told this May not be the fault and it could be any reason why the engine management light has re occurred .

I took the car yesterday to a local garage for a diagnostic check and the p0420 fault came up , there answer was to replace the cat ????

Where can I take the car to get the fault diagnosed correctly a new cat seems a little ott and why didn't the main dealership replace this sensor at the time , cleaning it seems stupidity .We live in North Yorkshire

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Hello sorry to here your troubles, 400 to clean the sensor is a bit steep, a new one should cost that much in the 1st place, best trying to source parts outside the dealer network www.importcarparts.co.uk is good and fairly close to you.

There seems to be a lot of differing info regarding the code, normally the downstream sensor located behind the cat is the culprit,

Have a look here for a bit more info


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To be honest I'm not sure about garages in leeds I take mine to an independent subaru garage in Sheffield.

There's a place in Bradford called Richard Henry motorsprt that is a subaru specialist also.

May be worth posting in the general section for help and garage advice as more people generally tend to browse there

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