OBD2 P0420 Error?

CRC987

MEMBER
2005 SUBARU OUTBACK
2.5L
4 CYL
4WD
AUTOMATIC
239,000 MILES

NAPA Authorized repair facility performed timing belt service. Tensioner failed and engine damaged. NAPA Store machine shop rebuilt long block. Repair facility completed engine assembly and installed. After driving, CEL and P0420 code appears. CA State specific Cat replacement installed. P0420 code returned within 100 miles. O2 sensors replaced twice. P0420 code remains. PCM replaced. Again 420 code appears. Could an error in reassembling engine or adjusting something on the engine incorrectly cause this?

Wednesday, July 30th, 2025 AT 12:07 PM

3 Replies

CANNON1349

MECHANIC
862 POSTS

Hello, the catalytic efficiency codes can be tricky. Usually they are a result of an issue causing the catalytic converter to be less efficient than it is intended to be, like when the engine is misfiring or burning oil, sending foreign material into the cat, etc. Sometimes it does point to a catalytic converter being worn out or used up, but not in a case like this. It would be important to know which parts from the old engine were used, as some of the parts like the manifolds can hold oil, soot, metal shavings etc. From a catastrophic failure and still cause a catalytic efficiency issue.

William

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Wednesday, July 30th, 2025 AT 6:35 PM

CRC987

MEMBER
2 POSTS

Thanks. I do smell oil (burnt oil?) After a long drive with the car before shutting off. And there is no sign of dripping oil anywhere and not on the exhaust. I will continue with your suggestion.

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Wednesday, July 30th, 2025 AT 10:48 PM

CANNON1349