Parts
$50–2000
P0420 — Catalyst Efficiency Below Threshold (Bank 1)
CAUTIONIs it safe to drive? Usually safe to drive short-term, but it won't pass emissions, and the cause (often a slow leak or upstream problem) can keep killing converters if ignored.
What P0420 means
The rear oxygen sensor is reading almost the same as the front one — meaning the catalytic converter isn't storing and converting oxygen like it should. The computer concludes the cat has lost efficiency.
Most likely causes (in order)
- A failing downstream O2 sensor reading falsely (check this BEFORE buying a cat)
- Exhaust leak before or near the rear O2 sensor
- Worn-out catalytic converter (real — but verify the cheap stuff first)
- An upstream misfire or rich/lean condition that damaged the cat
- Cheap aftermarket cat that never met spec
Symptoms you might notice
- Check-engine light (often the only symptom)
- Mild power / economy loss
- Rotten-egg smell if running rich
What to check first
A cat rarely dies on its own — something upstream usually killed it. Scan the rear O2: it should stay fairly steady while the front one switches. Find and fix exhaust leaks and any misfire/fuel-trim issue FIRST, then re-test before spending on a converter.
Repair cost & difficulty
Labor
0.5–3 hr
Difficulty
Moderate