Had a 2008 Civic in the bay last Tuesday with a persistent P0420 code, tried the old downstream O2 sensor spacer trick and it actually cleared after 50 miles.

3 comments

3 Comments

Has anyone checked the engine coolant temperature sensor data? I've seen a faulty ECT sensor mess with the fuel mixture enough to throw a P0420, because the car runs too rich or too lean and confuses the downstream O2 reading. @the_charles is totally right that the spacer is a band-aid for a bad cat, but if the cat is actually okay, a different sensor could be the real culprit. It's a cheap part to test and swap out before condemning the converter.

the_charles2mo ago

Remember that spacer is just a band-aid. It tricks the computer by moving the sensor out of the exhaust stream, so it reads cleaner. The real problem is usually a worn out catalytic converter on those older Civics. You might pass an emissions check for now, but if the cat is truly failing, the code will likely come back when the spacer gets gunked up or under heavier load. It's a cheap fix that can buy time, but don't be shocked if the light pops on again next year.

kevin_schmidt971mo ago

Holly's point about the ECT sensor is solid... but on those 8th gens, a slow upstream O2 sensor can do it too. The lazy switching doesn't heat the cat properly, so the downstream reads low efficiency. I've seen them pass the active test but fail in real driving, tricking you into replacing a good cat.