2002 - S - P0420?

subaruforester_power · 2025-10-12T09:32:26-0400

Friend of mine has a 2002 Forester and has this code and I can't remember 100% if we changed the front oxygen sensor maybe 8 or 9 years ago.. but I know we did the passenger front axle replacing the boot I was using a code reader to look at the live data and if I'm correct (foxwell reader) A/F sensor #1 is the front o2 sensor and Rear o/2 is the rear when revving the numbers for the rear went between 0.000v to like 0.789v

I didn't get around to checking the other things as it was getting a bit late but will again this coming weekend.

bman400 · 2025-10-12T11:44:09-0400

Could be: catalytic converter. P0420 means the ECU is comparing front O2 sensor with rear O2 sensor readings and thinks the Catalytic converter is not functioning.

The front sensor is primarily used to control A/F ratio. The rear maybe used for A/F.

You've confirmed the rear sensor is working because it's not flat lining.

ymerejeremy · 2025-10-13T11:00:53-0400

a gallon on lacquer thinner in the tank when kinda low should clear that up. it cleans the cat.

subaruforester_power · 2025-10-13T19:24:23-0400

Looks like he's going to go with replacing the front and rear o2 sensors.. kind of odd but I'm thinking with a bore hole camera I'd be able to see the front and rear of the cat.

bman400 · 2025-10-13T19:32:39-0400

Aftermarket O2 sensors are known to cause problems. OEM is the way to go.

subaruforester_power · 2025-10-13T19:36:57-0400

They never been changed before.

2002 - S - P0420?

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