OBD-II Code · Emissions
P0420
Catalyst System Efficiency Below Threshold (Bank 1)
Severity
medium
Worth booking. It costs fuel, emissions or comfort until it is fixed.
Can you drive it
Only if running normally
A short trip to a shop may be reasonable if the light is steady and the car runs normally. Stop safely for a flashing light, severe shaking, overheating, loss of power, or another urgent warning. The code alone cannot confirm safety.
Indicative cost (USD)
$100–$2,000
Varies by vehicle, diagnosis, parts, and local labor. This is not a repair quote.
New catalytic converter is expensive — diagnose for cheaper causes first.
The catalytic converter isn't reducing emissions efficiently. Could be the converter itself failing, or upstream issues like misfires damaging it.
How a shop works this
Work from evidence to part, not the other way round. A code names a circuit or a symptom; it does not name the part that failed.
Note every code that is set, not just this one.
Codes that appear together usually narrow the cause faster than any one of them read alone.
Read the freeze frame — the conditions the fault was recorded under.
Cold or warm, idle or load, road speed and engine speed. A fault that only appears cold is a different fault from one that only appears under load, even with the same code on the scanner.
Confirm the fault is current rather than history.
Clear the code and see whether it returns under those same conditions. A code that does not come back may have been logged during a repair, a flat battery, or a bad connection that has since been disturbed.
Then work the likely-causes list in the order shown.
That order is what a shop checks first for this code — wiring and connections before the component they run to, because a connector costs nothing to inspect and is a common answer.
If the code comes back after a repair, the underlying fault is still there. That is the point to bring in a scanner with live data and freeze-frame, or a shop that has one.
Common symptoms
- Check engine light
- Rotten egg smell from exhaust
- Reduced fuel economy
- Failed emissions test
Likely causes
- Failed catalytic converterMost common
- Leaking exhaust before converter
- Bad downstream O2 sensor
- Engine burning oil
- Misfires damaging converter
Read the full diagnostic procedureP0420 is the most-Googled OBD code in the world and also the one most likely to result in unnecessary catalytic converter replacement.
P0420 is the most-Googled OBD code in the world and also the one most likely to result in unnecessary catalytic converter replacement. The code says "bank 1's downstream O2 sensor doesn't see enough oxygen change after the cat" — which CAN mean the cat is dead, but more often means something cheaper is wrong: a small exhaust leak before the cat tricks the downstream sensor into reading the wrong oxygen level; a failing downstream O2 sensor itself reads stuck-rich; an oil-burning engine has glazed the cat with deposits that recover after a fresh oil change + a few highway drives.
The diagnostic order on a shop floor
- scan O2 sensor live data with the engine warm, looking for the downstream B1S2 to be relatively flat (cat working) or to oscillate just as much as the upstream B1S1 (cat failed or near-failed).
- If the downstream still oscillates, the cat is suspect but the sensor itself could be the cause — swap B1S2 with a known-good unit before condemning the cat.
Vehicle-specific patternsVehicle-specific patterns common in shop work: 2007-2015 Toyota Camry/Corolla with the 2AZ-FE 2.4L often throws P0420 because of the ECM's overly-agg…
- 2007-2015Toyota Camry/Corolla with the 2AZ-FE 2.4L often throws P0420 because of the ECM's overly-aggressive cat-monitoring threshold — Toyota issued a TSB extending the warranty on 2AZ catalytic converters to 15 years/150,000 miles in many markets; check that first.
- 2008-2013Honda Accord/CR-V/Pilot with the J35 V6 often throws P0420 on bank 1 specifically when the rear bank fuel trim drifts long-lean (vacuum leak at the intake manifold runner, NOT the cat).
- 2001-2007Ford Escape/Mazda Tribute with the 3.0L V6 frequently throws P0420 from cracked exhaust manifolds before the cat — the manifold leak is the cause, not the cat itself; replacing the cat without fixing the leak guarantees the code returns within 100 miles.
Estimated repair$0 (TSB-eligible) to $2,000+ (full replacement with OEM-grade cat + new downstream sensor).
Look up another code
P0420 — frequently asked
What does a P0420 code mean?
The catalytic converter isn't reducing emissions efficiently. Could be the converter itself failing, or upstream issues like misfires damaging it.
What causes a P0420 code?
Common causes: Failed catalytic converter; Leaking exhaust before converter; Bad downstream O2 sensor; Engine burning oil; Misfires damaging converter.
What are the symptoms of a P0420 code?
Typical symptoms: Check engine light; Rotten egg smell from exhaust; Reduced fuel economy; Failed emissions test.
Is it safe to drive with a P0420 code?
Only if running normally. A short trip to a shop may be reasonable if the light is steady and the car runs normally. Stop safely for a flashing light, severe shaking, overheating, loss of power, or another urgent warning. The code alone cannot confirm safety. Estimated repair: $100-$2,000. New catalytic converter is expensive — diagnose for cheaper causes first..
How do I diagnose a P0420 code?
Work from evidence to part, not the other way round. A code names a circuit or a symptom; it does not name the part that failed. 1. Note every code that is set, not just this one. 2. Read the freeze frame — the conditions the fault was recorded under. 3. Confirm the fault is current rather than history. 4. Then work the likely-causes list in the order shown.
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