Can you keep driving?
Can you keep driving?
Stop driving if any of these apply
- !The check-engine light is flashing.
- !The engine is running badly enough that traffic safety or engine damage becomes a concern.
What to check first
Step-by-step checks
- 1
Free - no tools
Check for misfire, lean, or fuel-trim codes that could explain why the converter test failed
- 2
Free - no tools
Inspect the exhaust for leaks ahead of the converter and make sure the engine is running normally
- 3
Basic tool needed
Look at upstream and downstream oxygen-sensor behavior if scan data is available
- 4
Basic tool needed
Do not replace the converter until the engine-side cause has been checked first
- 5
Basic tool needed
If the code appeared after another repair, confirm the original problem is really fixed
If the code returns
- -If the engine has recurring misfires or lean running, address that before condemning the converter.
- -If exhaust leaks or sensor faults are present, fix those and re-test before replacing major parts.
- -If the converter test still fails after the engine is healthy, the catalyst itself becomes a stronger suspect.
Background
What this code means
P0420 is a generic OBD-II catalyst-efficiency code.
The code does not automatically mean the converter is bad. Misfires, fuel-control problems, exhaust leaks, or sensor issues can all make the converter look inefficient.
Diagnosis
Common causes
Exhaust leak
A leak ahead of the converter can distort the oxygen-sensor readings.
Engine misfire
Unburned fuel entering the exhaust can damage or overwhelm the converter.
Fuel-control problem
Lean or rich running can make the catalyst test fail even if the converter is still intact.
Worn catalytic converter
If the engine and exhaust are otherwise sound, the converter itself may be tired.
Avoid these mistakes
What not to do
- xDo not replace major parts before checking the simple causes first.
- xDo not ignore a flashing check-engine light or obvious drivability symptoms.
Parts
Parts that may need replacing
See also
Related OBD codes
P0430
P0430 means the catalytic converter on bank 2 is not meeting efficiency expectations.
P0300
P0300 usually means the engine is detecting random or multiple-cylinder misfires.
P0171
P0171 usually means bank 1 is running too lean.
P0128
P0128 usually means the engine is not reaching the temperature the ECU expects in time.
Source notes
Prototype Ford overlay for catalyst-efficiency guidance.
- -On Ford applications, exhaust leaks and upstream engine-running faults still need to be ruled out before converter replacement.
This guide is written with Ford vehicles in mind, but bulletin history, engine variants, and component locations can still change the best repair path.
This is generic OBD-II guidance. Specific makes may have model-specific converter thresholds, oxygen-sensor tests, or service bulletins that should override generic advice.
Last reviewed: 2026-04-11