Catalyst System Efficiency Below Threshold (Bank 2)

What it means

The ECU compares the oxygen sensors before and after the catalytic converter on bank 2. A working converter stores oxygen, so the downstream sensor should be steady while the upstream one switches. If the downstream sensor starts copying the upstream one, the ECU decides the converter is no longer cleaning the exhaust well enough. The converter itself is often the cause, but not always: a lazy downstream sensor or an exhaust leak can trigger the same code.

Symptoms

• Check engine light on
• Usually no change in how the car drives
• Rotten egg (sulphur) smell
• Loss of power if the converter is clogged
• Rattle from under the car if the converter is broken inside

Probable causes

Most likely first

1. 1Worn-out catalytic converter (common past 160,000 km / 100,000 miles)
2. 2Converter damaged by an earlier misfire or rich condition
3. 3Exhaust leak near the downstream O2 sensor
4. 4Faulty downstream O2 sensor
5. 5Engine burning oil or coolant and poisoning the converter
6. 6Cheap aftermarket converter that does not meet the original specification

How to diagnose it

Work through these in order before buying parts

1. 1Record the code and its freeze frame data (speed, load, temperature when it set), then clear it and see if it comes back.
2. 2Fix any misfire, fuel trim or O2 sensor codes first. Replacing the converter without fixing the root cause will kill the new one too.
3. 3Graph the upstream and downstream O2 sensors for bank 2 with the engine fully warm at 2,500 RPM. A good converter keeps the downstream sensor fairly steady (around 0.6–0.7 V); a failed one makes it switch like the upstream sensor.
4. 4Check for exhaust leaks between the engine and the downstream sensor.
5. 5Check for oil burning (blue smoke, oil use) or coolant loss.
6. 6Measure temperature before and after the converter with an infrared thermometer: after should be hotter on a working converter.
7. 7Check for technical service bulletins (TSBs) or software updates for your make and model; some codes have known factory fixes.

Common fixes

• Repair exhaust leaks
• Replace the downstream O2 sensor if it is lazy
• Fix oil burning or coolant leaks
• Replace the catalytic converter with an OE-grade or legally compliant unit