P0420
Catalyst System Efficiency Below Threshold (Bank 1)
The engine computer compares oxygen sensor readings before and after the catalytic converter. P0420 means the bank 1 catalytic converter is not reducing emissions well enough, or the sensors are making it look that way. Common causes include a worn catalytic converter, exhaust leak, failing oxygen sensor, or engine problem that damaged the converter.
- SEV
- 3/5
- DRIVE
- CAUTION
- DIY
- $20–$1,500
- SHOP
- $150–$2,500
Quick answer
AI-CITATION READYWhat it means
Can you drive with it?
Most common causes
- Catalytic converter worn out, contaminated, overheated, or internally damaged
- Exhaust leak near the manifold, flex pipe, converter, or oxygen sensor bung
- Rear downstream oxygen sensor slow, biased, damaged, or reading incorrectly
Typical repair cost
DIY usually runs $20–$1,500. Typical shop repair lands around $150–$2,500, depending on the root cause.
01 / Definition
P0420 means the powertrain control module has detected catalytic converter efficiency below the calibrated threshold on bank 1. It usually happens when the downstream oxygen sensor signal is too similar to the upstream oxygen sensor signal, indicating the converter is not storing oxygen and processing exhaust gases correctly. The root cause may be the converter itself, an oxygen sensor problem, exhaust leaks, fuel trim issues, oil or coolant contamination, or unresolved misfires.
02 / Drive status
With caution. You can usually drive short distances with P0420 if the vehicle runs normally. Do not ignore flashing check engine lights, misfires, rotten-egg smells, overheating, or major power loss because those can damage the converter or make the car unsafe.
03 / Symptoms
- Check engine light
- Failed emissions test
- Reduced fuel economy
- Rotten egg sulfur smell from exhaust
- Rattling noise from catalytic converter
- Loss of power if converter is restricted
- Vehicle may run normally with no obvious symptoms
04 / Causes
| 1 | Catalytic converter worn out, contaminated, overheated, or internally damaged | high |
|---|---|---|
| 2 | Exhaust leak near the manifold, flex pipe, converter, or oxygen sensor bung | medium |
| 3 | Rear downstream oxygen sensor slow, biased, damaged, or reading incorrectly | medium |
| 4 | Engine misfire, rich mixture, oil burning, or coolant burning damaged the converter | medium |
| 5 | Front upstream oxygen sensor or air-fuel ratio sensor reporting incorrect data | low |
| 6 | Aftermarket converter below required efficiency for the vehicle calibration | low |
05 / Diagnostic sequence
- 01Check for other codes first, especially misfire codes, oxygen sensor codes, fuel trim codes, or coolant temperature codes.
- 02Inspect the exhaust system for leaks, cracks, loose flanges, damaged flex pipe, or missing hardware before and near the catalytic converter.
- 03Use live scan tool data to compare upstream and downstream oxygen sensor activity after the engine is fully warm.
- 04Check fuel trims and misfire counters to make sure the engine is not running rich, lean, or misfiring.
- 05Inspect for oil or coolant consumption that can contaminate and damage the catalytic converter.
- 06If the converter rattles, overheats, smells strongly of sulfur, or causes high exhaust backpressure, test for internal damage or restriction.
- 07Verify the correct OEM-quality or emissions-compliant replacement converter is used if replacement is needed.
- 08Clear the code after repairs and complete the required drive cycle to confirm the catalyst monitor passes.
06 / Repairs
| 1 | Repair exhaust leaks before or near the catalytic converter and oxygen sensors | $20–$500 |
|---|---|---|
| 2 | Fix misfires, rich running, oil burning, coolant burning, or fuel trim problems before replacing the converter | $50–$1,500 |
| 3 | Replace a faulty downstream oxygen sensor only if testing shows the sensor is inaccurate | $80–$350 |
| 4 | Replace the catalytic converter with a correct emissions-compliant part if converter efficiency is confirmed low | $400–$2,500 |
| 5 | Update powertrain control module software if a manufacturer bulletin applies | $0–$200 |
MFG / Manufacturer notes
These supplements add make-specific diagnostic framing. Pages without full matrix backing or lane approval stay guarded and canonicalize back here until they are explicitly approved for indexing.
07 / Related codes
08 / FAQ
What does P0420 mean?
P0420 means the bank 1 catalytic converter is not meeting the efficiency target expected by the engine computer.
Can I drive with P0420?
Usually yes for short trips if the vehicle runs normally, but repair it soon. Stop driving if there is a flashing check engine light, strong sulfur smell, overheating, severe misfire, or major power loss.
Does P0420 always mean I need a catalytic converter?
No. A converter is common, but exhaust leaks, oxygen sensor problems, fuel mixture issues, and misfires should be checked first.
Will replacing the oxygen sensor fix P0420?
Only if testing proves the sensor is wrong. Replacing oxygen sensors without diagnosis often does not fix P0420.
Can a bad spark plug cause P0420?
A bad spark plug can cause misfires that overheat and damage the catalytic converter. Fix misfires before replacing the converter.
09 / Source and method
- DATA BASIS
- OBD-II REFERENCE + OBD2.HELP
- METHOD
- STATIC VALIDATION
- SAFETY
- INFORMATIONAL
This page combines OBD-II diagnostic reference data with OBD2.help generated diagnostic guidance for code meaning, likely causes, and repair direction.
Publishing uses deterministic schema and build validation, plus manual spot checks on representative pages before release.
Safety-critical diagnosis and repairs should be confirmed with a qualified mechanic, especially when the vehicle is misfiring, overheating, or losing power.