Emission codes
P0420: catalyst system efficiency below threshold (bank 1)
P0420 is one of the most feared codes because a catalytic converter is expensive. It does not always mean a new converter, and when it does, finding what wore the converter out is part of the repair.
P0420: can I drive, and what will the labor cost?
Estimated labor, replace downstream oxygen sensor (bank 1 sensor 2)
$33 to $110
| Labor time (estimate) | 0.3 to 1 h |
| Rate used | $110/h |
| Technician wage, United States (average) | $27.13/h |
| Moderate: fix within weeks Drivable; the car will fail an OBD emissions inspection, and the cause behind a worn converter should be found first. | |
Labor only: parts and the diagnostic fee are not included. Hours are our estimate for a typical car; book times vary by model. Wage: BLS, May 2025.
P0420 is the generic OBD-II code for "catalyst system efficiency below threshold, bank 1". A working converter stores oxygen and releases it as the mixture swings slightly rich and lean, so the oxygen sensor behind it reads a steady, flat signal while the sensor in front switches back and forth. As the converter ages, its storage shrinks, and the downstream signal starts to copy the upstream one. The computer compares the two and sets P0420 when the converter is judged below the threshold set in the emission rules. The most frequent fix is the converter itself, worn by age or by an engine fault. A downstream oxygen sensor that reads wrong, an exhaust leak near the sensors, and engine problems that damaged the converter, misfire, oil burning or coolant loss, follow. The car drives normally but fails an OBD inspection. A converter is covered by the federal emission warranty for 8 years or 80,000 miles (40 CFR 85.2103).
Is this repair still under the emission warranty?
Catalytic converter
Likely covered: ask a dealer
| Federal emission warranty, 8 years or 80,000 miles | within |
Age counts from delivery to the first buyer; whichever limit comes first ends the warranty. Your warranty booklet lists the covered parts and prevails.
Codes, rules and labor data checked by Radif Partners · Editorial policy · Method and sources
P0420 at a glance
| Code | P0420 |
| Standard description | Catalyst System Efficiency Below Threshold (Bank 1) |
| System | Exhaust emission controls |
| Severity | Moderate: fix within weeks |
| Common symptoms | usually none, rotten-egg smell, rattle from the converter, emissions test failure |
Find the reason before buying a converter
Converters rarely die alone. Misfires (P0300) send raw fuel into them, which burns and melts the substrate. An engine that burns oil or leaks coolant coats them. A rich mixture (P0172) overheats them. Fit a new converter to an engine with any of these faults and it can fail again within months. A good shop checks fuel trims, misfire counts and oil consumption first.
When the converter is fine
- A downstream sensor that is lazy or biased can make a healthy converter look worn; it is the cheap thing to rule out, and its heater code is P0141.
- An exhaust leak between the sensors lets in outside air and confuses the comparison.
- Engine software updates have changed the converter test on some models; a dealer can say whether one applies.
What replacement involves
Labor is modest for a bolt-in converter, 1 and 2.5 hours, and longer when it is built into the exhaust manifold. In California and states that adopted its rules, a replacement converter must be approved by CARB for the vehicle, as the CARB fact sheet explains, and a cheaper universal part may not be legal there. The warranty check on this page tells you whether the 8-year, 80,000-mile coverage still applies, and California adds longer limits for high-priced parts.
Disabling the converter, or fitting a device that fools the downstream sensor, is illegal on a road vehicle and does not pass a properly conducted inspection.
Likely causes of P0420, in order
| Cause | How often |
|---|---|
| Catalytic converter worn out by age or by an engine fault | most frequent |
| Downstream oxygen sensor reading wrong | frequent |
| Exhaust leak near the sensors | frequent |
| Misfire, oil burning or coolant loss that damaged the converter | frequent |
| Engine computer software needing an update | less frequent |
P0420 repairs and labor time
| Repair | Labor (estimate) | At $110/h |
|---|---|---|
| Replace downstream oxygen sensor (bank 1 sensor 2) | 0.3 to 1 h | $33 to $110 |
| Repair exhaust leak near the sensors | 0.5 to 1.5 h | $55 to $165 |
| Replace catalytic converter (direct-fit) | 1 to 2.5 h | $110 to $275 |
| Replace converter built into the exhaust manifold | 2.5 to 5 h | $275 to $550 |