P0420 – Catalyst System Efficiency Below Threshold (Bank 1)
What does P0420 mean?
The engine computer compares the upstream (pre-catalyst) and downstream (post-catalyst) oxygen sensor signals to judge how well the catalytic converter is working. When the downstream sensor starts mirroring the upstream sensor too closely, it means the converter is no longer storing and releasing oxygen effectively, and P0420 is set for Bank 1 — the side of the engine containing cylinder #1.
What causes P0420?
Listed from most to least common:
- 1Worn or failed catalytic converter (most common on higher-mileage vehicles)
- 2Aging or 'lazy' downstream oxygen sensor reporting inaccurate data
- 3Exhaust leak upstream of or near the converter, letting extra oxygen into the stream
- 4Engine misfire or rich/lean fuel condition that has damaged the converter
- 5Leaking fuel injector or failing upstream O2 sensor skewing fuel trim
- 6Aftermarket or non-OEM converter that does not meet the vehicle's monitoring thresholds
Code breakdown
Common symptoms
- Check engine light on, often with no change in how the car drives
- Failed emissions or smog test
- Slightly reduced fuel economy in some cases
- Sulfur or 'rotten egg' smell from the exhaust when the converter is failing
How to diagnose
- 1Scan for additional codes first — misfire (P0300 series) or fuel trim codes (P0171/P0172) must be fixed before diagnosing P0420.
- 2Graph the upstream and downstream O2 sensor voltages with a live-data scanner. The upstream should fluctuate rapidly; the downstream should stay relatively flat and slow.
- 3If the downstream sensor mirrors the upstream sensor's rapid switching, converter efficiency is genuinely low.
- 4Inspect the entire exhaust system ahead of and around the converter for leaks, cracks, and bad gaskets. A small leak can trigger this code with a healthy converter.
- 5Check fuel trims. Long-term fuel trim well outside roughly ±10% points to a fuel or air issue that needs fixing first.
- 6Compare converter inlet and outlet temperature after warm-up. The outlet is normally hotter on a working converter.
The car usually drives normally, but it will fail an emissions test and a genuinely failed converter can worsen fuel economy over time. It is not an emergency, but it should not be ignored indefinitely.
Common fixes and estimated costs
| Repair | Estimated cost |
|---|---|
| Repair exhaust leak (gasket, clamp, or weld) | $50–$250 |
| Replace downstream oxygen sensor | $120–$350 |
| Replace catalytic converter (aftermarket) | $400–$1,200 |
| Replace catalytic converter (OEM) | $900–$2,500 |
Costs are rough US estimates including parts and labor. Actual pricing varies significantly by vehicle, region, and shop rates.
Good to know
Do not replace the converter first — it is the most expensive part and often not the actual fault. Verify with live O2 data. Note that many Toyota and Honda vehicles will re-set P0420 unless an OEM converter is used, because aftermarket units often fail to meet the factory monitoring threshold.
Related codes
This information is provided for general reference only and may vary by manufacturer, model, and year. It is not a substitute for professional diagnosis or repair.