✅ P0420 on a 2004 Civic with 210k. Cat or O2 sensor? Not spending $900 on a cat without evidence.

Oct 19, 2026 · 2.2k views

Code came back twice after clearing. No driveability issues, no smell, passes visual. Downstream O2 was replaced 3 years ago. What is the actual diagnostic here instead of guessing?

3 replies

✅ Accepted solution

@spokes_n_sprockets 75 karma · 1d ago

Before you buy: a lot of Hondas that age have a tiny leak at the flex pipe or the manifold-to-cat gasket that pulls in air and fakes a dead cat. Smoke test the exhaust or spray soapy water with it running. Cheap test.

@torque_tina OP 131 karma · 1d ago

Update: graphed both O2 sensors on the scan tool. Downstream is mirroring upstream, which per ScannerDanner means the cat is not storing oxygen. Checked for exhaust leaks first (none). Looks like it really is the cat.

@benchtop_bea 115 karma · 1d ago

Also check the aftermarket-cat situation in your state. In California only CARB-legal cats pass, which is a big chunk of the $900.