P0420: Catalyst Efficiency Below Threshold (Bank 1)
Key Takeaway
P0420 is one of the most common — and most misdiagnosed — check engine codes. Here's how to avoid a $2,500 mistake.
What P0420 Means
P0420 indicates the rear O2 sensor detects that the catalytic converter on Bank 1 is not converting exhaust pollutants efficiently enough to meet emissions standards.
Is It the Catalytic Converter or the O2 Sensor?
| Factor | Catalytic Converter | O2 Sensor |
|---|---|---|
| Repair cost | $1,000 – $2,500 | $150 – $300 |
| Frequency | ~60% of P0420 cases | ~40% of P0420 cases |
| Additional symptoms | Rotten egg smell, power loss | None usually |
| Diagnosis method | Compare upstream vs downstream O2 | Scope test response time |
How to Tell the Difference
Compare front (upstream) and rear (downstream) O2 sensor waveforms:
Can You Drive With P0420?
Yes, short-term. P0420 is an emissions efficiency code and won't cause immediate engine damage. However, it will fail emissions inspections.
ProHow Steer Helps
Steer reads P0420, monitors O2 sensor trends, and helps you determine if it's the cat or the sensor before you spend thousands.
Fix Any Engine Issue with Steer
Get instant plain-English explanations for any engine code on your iPhone. Steer stays connected 24/7 to monitor your car's health.
