The pre-catalyst (upstream) sensor cycles continuously as the ECU closed-loops on mixture. The post-catalyst (downstream) sensor should be far more stable — the catalyst smooths oxygen concentration. The ratio of their activity is a direct catalyst health indicator.
Activity interpretation
- Downstream flat, upstream cycling: healthy catalyst, storing oxygen.
- Downstream mirroring upstream: catalyst storage lost (or downstream sensor misfuelled).
- Both sluggish: consider fuel/oil contamination on both sensors.
- Downstream stuck: cold sensor, heater circuit failure, or sensor death.
Heater circuit
O₂ sensors need heated ceramic to read correctly. A slow warm-up or heater DTC means the sensor is not in its operating window — and any activity measurement is invalid.
Fuel-type dependency
E85 and LPG produce different expected activity patterns than E10 petrol due to combustion water content and adiabatic flame temperatures. Account for fuel when judging activity ratios.