P0420 is the most-searched OBD-II code in the United States. If your check engine light is on and a scanner pulls this code, here's everything you need to know.
What P0420 actually means:
The code stands for "Catalyst System Efficiency Below Threshold (Bank 1)." Your vehicle's ECU monitors the efficiency of the catalytic converter by comparing the signal from the upstream (before the cat) and downstream (after the cat) oxygen sensors. When the downstream sensor's readings look too similar to the upstream sensor — meaning the cat isn't cleaning exhaust gases — P0420 is stored.
Is P0420 serious?
Medium urgency. The car is safe to drive short-term — the code won't cause sudden breakdown or immediate mechanical damage. However, you'll fail an emissions test in most states, and if the root cause is engine misfires contaminating the converter, the damage can compound over time.
The three most common causes:
- →Worn catalytic converter — Most common in vehicles over 100k miles. The precious metal catalyst (platinum, palladium, rhodium) degrades over time. No repair — only replacement.
- →Failed oxygen sensor — A faulty upstream O2 sensor sends incorrect data to the ECU, which can trigger a false P0420. Much cheaper fix at $120–$300.
- →Exhaust leak — A small leak before the downstream O2 sensor introduces fresh oxygen, making the sensor read incorrectly.
Repair cost breakdown:
- →O2 sensor replacement: $120–$300
- →Aftermarket catalytic converter: $400–$900
- →OEM catalytic converter: $800–$2,200
- →Labor: $100–$250 (typically 1–2 hours)
California residents: Note that California has stricter CARB regulations on catalytic converters. Make sure any replacement part is CARB-compliant, or you'll fail the smog check anyway.