P0420 – Catalyst efficiency below threshold – bank 1
P0420 means the monitored catalyst efficiency on bank 1 is below the required threshold. It reports an emissions-monitoring result, rather than a complete diagnosis of the catalytic converter.
Powertrain · Exhaust aftertreatment
Open this code in the decoderMeaning and affected system
The engine controller evaluates catalyst operation using exhaust measurements. Oxygen storage is part of the monitoring methods described by Ford. Test conditions and calculations depend on the engine. Bank 1 is the cylinder bank containing cylinder 1; its physical location must be checked for the particular vehicle.
Possible causes
- An aged or damaged catalytic converter is one possibility.
- Exhaust leaks and implausible oxygen-sensor signals are influences to investigate.
- Accompanying fuel-mixture or misfire codes can be important to the investigation.
Possible symptoms
- An illuminated check engine light may be the only noticeable sign.
- P0420 alone does not establish a loss of power or a particular driving symptom.
Useful diagnostic considerations
- Save all codes and freeze-frame information before clearing the memory.
- Have the exhaust and sensor behaviour checked under the manufacturer’s test conditions. One voltage reading is insufficient.
- Assess related faults first, then establish whether the catalyst monitor still reports a problem after they are resolved.
What this code does not prove
P0420 does not instruct you to replace both oxygen sensors. A sensor-circuit fault and a catalyst-efficiency result describe different problems.
Related fault codes
Sources and interpretation
This explanation was editorially cross-checked against the technical sources below. Manufacturer documents concern particular vehicle applications; their thresholds and repair procedures must not be applied universally.
- Ford: 2013 MY OBD System Operation Summary for Gasoline Engines
- HELLA: Checking the lambda sensor — diagnosis and troubleshooting
OBD-II code data: OBDex (CC0 1.0) · 196fddffb6ef
Editorial source review: .
The causes listed are possibilities without a likelihood ranking. This page cannot replace diagnosis of the vehicle. A code alone confirms neither a failed component nor that continuing to drive is safe. The actual condition, warnings, handbook and applicable manufacturer diagnostics determine the next step.