Catalyst System Efficiency Below Threshold (Bank 2)
What it means
This code appears on engines with two banks, such as V6 and V8 engines, and means the catalytic converter on the bank without cylinder 1 is no longer cleaning the exhaust as it should. The computer sees this when the oxygen sensor readings before and after the converter look alike.
Related lights
Dashboard lights that may come on with this code.
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 bank 2 catalytic converter.
- Faulty or slow bank 2 downstream oxygen sensor.
- Exhaust leak near the bank 2 sensors.
- Misfires or a rich mixture in bank 2 cylinders that damaged the converter.
- Oil burning or coolant entering the exhaust and contaminating the converter.
How to diagnose5
- Fix any misfire or bank 2 fuel mixture codes such as P0174 first.
- Compare the bank 2 downstream sensor signal with the upstream one; similar switching means a weak converter.
- If P0420 is also stored, look for a shared cause such as oil burning or engine-wide misfires.
- Check the exhaust for leaks ahead of the downstream sensor.
- Confirm the downstream sensor is healthy before condemning the converter.
How to fix5
- Repair any exhaust leak.
- Replace the bank 2 downstream oxygen sensor if it is proven faulty.
- Fix the cause of misfires or oil burning before replacing the converter.
- Replace the bank 2 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.
In the Saudi market: SAR 250–6,000 (≈ USD 67–1,600)
Our prices come from the Saudi market, converted at a fixed rate (SAR 3.75 per USD). Prices in your country may differ a lot, so we show the cost level.
The cost starts with an exhaust leak repair or sensor, and reaches the upper end when the converter is replaced; prices vary widely between genuine and aftermarket parts.
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
Where is bank 2?
It is the side of the engine that does not contain cylinder 1. Its location varies between vehicles, so check the service information before replacing parts.
P0420 and P0430 are both stored. What does that mean?
Both converters failing together points to a shared cause such as oil burning, misfires or high mileage, so fix the cause before replacing both converters.
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