P0421: Warm Up Catalyst Efficiency Below Threshold (Bank 1)
Can you still drive? Yes, the vehicle is safe to drive. However, continued driving may worsen catalyst deterioration and you will fail emissions testing.
What Does P0421 Mean?
Code P0421 indicates that the warm-up catalytic converter on bank 1 is not reaching efficient operation quickly enough after engine start. The ECM monitors converter efficiency using the downstream oxygen sensor and has determined the converter is not performing within specifications during the warm-up phase. This is similar to P0420 but specifically relates to the warm-up period.
Need to read your codes?
An OBD-II scanner lets you read and clear diagnostic trouble codes from your vehicle's computer. Most scanners work with any car made after 1996.
Browse OBD-II Scanners on AmazonCommon Causes
- Deteriorated warm-up catalytic converter
- Faulty downstream oxygen sensor on bank 1
- Exhaust leak before the catalytic converter
- Engine running rich during warm-up phase
- Coolant temperature sensor giving incorrect readings
Symptoms
- Check engine light on
- Failed emissions test
- Reduced fuel efficiency
- Slight sulfur smell from exhaust
Diagnostic Steps
- 1Read and record freeze frame data for conditions during fault
- 2Monitor upstream and downstream O2 sensor waveforms during warm-up
- 3Check for exhaust leaks at the manifold and downpipe
- 4Inspect the catalytic converter for physical damage
- 5Verify coolant temperature sensor accuracy
- 6Compare downstream O2 sensor readings to upstream during warm-up cycle
Recommended Parts & Tools
catalytic converter
oxygen sensor
Estimated Repair Cost
| Type | Estimated Cost |
|---|---|
| Total Repair Cost Range | $200 - $2000 |
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Frequently Asked Questions
What is the difference between P0420 and P0421?
P0420 refers to general catalyst efficiency, while P0421 specifically monitors the warm-up catalyst performance during the initial minutes after engine start. Some vehicles have a separate warm-up catalyst closer to the engine.
Can I drive without fixing P0421?
Yes, you can drive normally. The main concern is increased emissions and failing an emissions test. The catalytic converter may continue to degrade over time if the root cause is not addressed.
Community Fix Reports
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