Warm Up Catalyst Efficiency Below Threshold (Bank 1)
What it means
The engine computer has detected that the warm-up catalyst on bank 1, the converter designed to heat up quickly after a cold start and usually the one closest to the engine, is no longer cleaning the exhaust efficiently. It judges this by comparing the oxygen sensor readings before and after the converter.
Symptoms4
- Check engine light on, often with no other symptoms.
- Occasional sulfur (rotten egg) smell from the exhaust.
- Power loss if the converter is clogged (less common).
- Failed emissions test.
Possible causes5
- Aged or internally damaged warm-up catalytic converter.
- Faulty or slow oxygen sensor after the converter.
- Exhaust leak near the oxygen sensors.
- Past misfires or a rich mixture that overheated the converter.
- Oil burning or coolant entering the exhaust and contaminating the converter.
How to diagnose5
- Fix any misfire or fuel mixture codes first, since they will ruin a new converter.
- Watch the signal of the sensor after the converter; if it switches like the upstream sensor, the converter is weak.
- Check the exhaust for leaks ahead of the sensors.
- Confirm the downstream sensor is healthy and responsive before condemning the converter.
- Ask about oil consumption or coolant loss to find what contaminated the converter.
How to fix5
- Repair any exhaust leak.
- Replace the oxygen sensor if it is proven faulty.
- Fix the cause of oil burning or misfires before replacing the converter.
- Replace the warm-up catalytic converter with a unit that meets the vehicle specification.
- Clear the codes and drive until the converter self-test completes.
Repair cost
In Saudi riyals; the highlighted part is the expected range for this fault.
The cost starts with an exhaust leak repair or oxygen sensor, and reaches the upper end when the converter is replaced, especially if it is built into the exhaust manifold.
Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Prices last updated: September 2026
Frequently asked questions
What is the difference between P0420 and P0421?
P0420 covers overall catalyst system efficiency on bank 1, while P0421 covers the warm-up converter that heats quickly after a cold start. How each is used depends on the vehicle design.
Must I replace the converter right away?
No. Check the oxygen sensor and exhaust leaks and fix any misfire first, because replacing the converter without fixing the cause will damage the new one.
This guide is for information only and does not replace an inspection by a qualified technician. If the fault involves brakes, steering, engine temperature, electrics or refrigerant, do not attempt the repair yourself.
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