What does P0420 Catalyst System Efficiency Below Threshold (Bank 1) mean?
The P042 code, also known as "Catalyst System Efficiency Below Threshold (Bank 1)", is a common diagnostic trouble code (DTC) that applies to all makes and models of vehicles from 1996 and newer.
It indicates that the catalytic converter, which is a part of the exhaust system, is not functioning as efficiently as it should be in reducing emissions from the exhaust.
The catalytic converter has two oxygen sensors, one in front and one behind it.
When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor reading should fluctuate, while the downstream oxygen sensor reading should be steady.
If these readings are too similar, it triggers the P042 code and illuminates the Check Engine Light.
It's important to note that catalytic converters are not typically a "wear" type item and may fail due to other underlying issues.
Symptoms to the driver may include the illumination of the malfunction indicator lamp (MIL), but there may not be noticeable drivability problems.
Possible causes for the P042 code can include using leaded fuel instead of unleaded fuel, a damaged or failed oxygen sensor, damaged or improperly connected downstream oxygen sensor wiring, a malfunctioning engine coolant temperature sensor, a damaged or leaking exhaust manifold, catalytic converter, muffler, or exhaust pipe, a failed or underperforming catalytic converter, retarded spark timing, or leaking fuel injector or high fuel pressure.
Some possible solutions for troubleshooting and fixing the P042 code include repairing any exhaust leaks, diagnosing the oxygen sensor operation using a scope, inspecting and replacing the downstream heated oxygen sensor if necessary, replacing the catalytic converter, and checking for proper exhaust temperature before and after the converter.
It's important to properly diagnose the issue to avoid wasting money on unnecessary parts replacements.
If the catalytic converter needs to be replaced, it is recommended to use an original manufacturer brand unit or a high-quality replacement part to prevent the code from returning.
Additionally, some vehicle manufacturers may offer a longer warranty on emissions-related parts, so it's worth checking into if applicable.
P0420 Symptoms
- Illumination of the MIL (malfunction indicator lamp)
- Feeling of reduced power output from the vehicle
Possible P0420 Causes
- Leaded fuel was used where unleaded was called for
- Damaged or failed oxygen/O2 sensor
- Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
- Engine coolant temperature sensor not working properly
- Damaged or leaking exhaust manifold/catalytic converter/muffler/exhaust pipe
- Failed or underperforming catalytic converter
- Retarded spark timing
- Oxygen sensors in front and behind the converter reporting too similar of readings
- Leaking fuel injector or high fuel pressure
- Cylinder misfire
- Oil contamination
Troubleshooting P0420 error code
- Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
- Use a scope to diagnose the oxygen sensor operation. Inspect the downstream heated oxygen sensor, replace if necessary.
- Replace the catalytic converter.
- Check the engine coolant temperature sensor and replace if necessary.
- Check the spark timing and adjust if necessary.
- Check for leaking fuel injectors and test fuel pressure.
- Check for cylinder misfires and perform necessary repairs.
- Inspect the engine oil for contamination and replace if necessary.
