| Cause | Prior | Repair |
|---|---|---|
| Downstream O2 sensor (bank 1, sensor 2)
Sensor 2 voltage should be slow and steady near 0.6-0.7V. If it mirrors sensor 1, the sensor is reporting, not the converter. |
30% | $217 |
| Catalytic converter genuinely failed
Only after the four above are excluded. Confirm with a temperature differential across the converter body. |
21% | $1,492 |
| Exhaust leak before or at the converter
Smoke test the exhaust. A leak upstream pulls false air across sensor 2. |
19% | $244 |
| Upstream O2 sensor lazy (bank 1, sensor 1)
Sensor 1 should oscillate 0.1-0.9V several times per second at cruise. A slow sensor drives the converter rich. |
16% | $231 |
| Engine running rich (injector or fuel pressure)
Negative long-term fuel trims. Fix the cause before replacing the converter or you will kill the new one too. |
14% | $142 |