Catalyst System Efficiency Below Threshold (Bank 1)
In short
P0420 means your engine computer thinks the catalytic converter on Bank 1 isn't cleaning exhaust gases efficiently enough. The most common real-world cause is a worn catalytic converter or a lazy downstream (rear) oxygen sensor — not always the expensive cat itself. It's safe to drive short-term, but it will fail an emissions test.
Is it safe to drive with P0420?
Yes, generally safe to drive for weeks. The car won't be damaged by the code itself. The risk is failing a smog/emissions test and gradually worse fuel economy. If you also have a misfire or rich-running code, fix that first — raw fuel destroys catalytic converters.
Symptoms
- Check engine light on (often the only symptom)
- Failed emissions / smog test
- Slight drop in fuel economy
- Rotten-egg (sulfur) smell from exhaust in some cases
- Rarely: mild loss of power if the converter is physically clogged
Common causes (most → least likely)
How to diagnose it (before buying parts)
- 1 Scan for OTHER codes first. Misfire (P030x) or fuel-trim (P0171/P0172) codes must be fixed before condemning the catalytic converter.
- 2 Check for exhaust leaks around the O2 sensors — a hiss or soot trail near the bungs is a giveaway.
- 3 Use a scan tool with live data to graph the downstream O2 sensor. A healthy cat makes the rear sensor read a fairly steady voltage; a worn cat makes it mimic the front sensor's rapid switching.
- 4 Compare front vs rear O2 sensor activity — this is the single best test to tell a bad cat from a bad sensor without throwing parts at it.
- 5 Only after the above point to the converter, replace the cat (OEM or CARB-compliant in emissions states).
Repair options & cost
By manufacturer
Extremely common on 2007-2011 Camry/Corolla and many V6s. Often a genuine worn OEM cat; aftermarket cats frequently re-trigger P0420, so OEM/Denso is the reliable fix.
Often the rear O2 sensor rather than the cat. Check the sensor with live data before buying a converter.
Check for exhaust manifold cracks and leaks at the flex pipe before condemning the converter.
Frequently asked questions
Can I drive with a P0420 code?
Yes, for short-term driving it's generally safe — the code won't damage your engine. You'll fail an emissions test and may lose a little fuel economy. Fix any misfire or rich-running codes promptly, as those actively destroy the catalytic converter.
Will a new O2 sensor fix P0420?
Sometimes. If the downstream oxygen sensor is lazy or faulty, replacing it clears the code — and it's far cheaper than a converter. Confirm with live data before assuming it's the cat.
How much does it cost to fix P0420?
Anywhere from ~$150 for an O2 sensor or exhaust-leak repair to $400-$2,200 for a catalytic converter, depending on the vehicle and whether you use OEM parts.
Will clearing the code make it go away?
Only temporarily. If the underlying issue isn't fixed, P0420 returns after a few drive cycles — and clearing it resets emissions monitors, which can itself cause a smog-test failure.