Had a customer bring in a 2008 Civic last Tuesday with a persistent P0420 code that had stumped two other shops.

They'd already replaced the cat and both O2 sensors. I spent about an hour just looking over everything, and finally spotted a tiny, almost invisible crack in the exhaust manifold right before the primary O2 sensor. It was letting in just enough air to throw the readings off. A $40 gasket and some time with a torque wrench later, and the code cleared for good. It was one of those small victories that just makes the whole week feel better. Anyone else have a recent 'aha' moment with a tricky sensor issue?

3 comments

3 Comments

skylerp311mo ago

That crack might not be the real fix, seanlee has a point about drive cycles.

walker.cole1mo ago

Glad you fixed it, but I've seen that code come back even after fixing exhaust leaks.

seanlee1mo ago

Figure out what else could be throwing that sensor off. A bad O2 sensor itself is common, or maybe a vacuum leak they missed. Sometimes the fix is right but the computer needs a few drive cycles to clear its mind. It's frustrating when a code just comes back for no clear reason.