Catalyst System Efficiency Below Threshold (Bank 1)
What it means
The engine computer compares the oxygen sensor readings before and after the catalytic converter. When the two readings look alike, the converter is no longer storing oxygen and cleaning the exhaust as it should, so this code is set.
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 catalytic converter.
- Faulty or slow downstream oxygen sensor.
- 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 cause this code and will ruin a new converter.
- Watch the downstream oxygen sensor signal; if it switches like the upstream one, the converter is weak.
- Check the exhaust for leaks ahead of the downstream sensor.
- Measure converter inlet and outlet temperatures; a working converter runs hotter at the outlet.
- Confirm the downstream sensor is healthy before condemning the converter.
How to fix5
- Repair any exhaust leak.
- Replace the downstream oxygen sensor if it is proven faulty.
- Fix the cause of oil burning or misfires before replacing the converter.
- Replace the 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 oxygen sensor, and reaches the upper end when the converter is replaced; converter 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
Do I have to replace the catalytic converter right away?
No. Check the downstream oxygen sensor and exhaust leaks first and fix any misfire, because replacing the converter without fixing the cause will damage the new one.
Can I drive with this code?
Usually yes, unless there is clear power loss or a burning smell, but repair it soon to avoid failing an emissions test.
Do exhaust cleaner additives fix it?
They may hide the code for a while if the converter is only dirty, but they cannot repair a damaged converter.
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