Fault Tree: P013x O2 / Lambda heater & circuit

Decision flow

1. Identify which sensor & which fault
   • Bank 1 vs Bank 2, Sensor 1 (upstream) vs Sensor 2 (downstream).
   • Heater circuit codes usually set quickly after start.
2. Check heater fuse / relay and power feed
   • With key on (or engine running), confirm heater feed voltage at the sensor connector.
   • If no power: fuse/relay/wiring back to supply.
   Use: Wiring diagrams basics →
3. Check heater ground control
   • Some ECUs ground-switch the heater. Check ground path and connector pin fit.
   • Do a voltage drop test under load rather than a simple continuity test.
   Use: Voltage drop testing →
4. Inspect harness routing near exhaust
   • Melted insulation, chafing, or stretched wires are common.
5. Rule out exhaust leaks (especially upstream sensor)
   • Leaks before the upstream sensor can pull in air and skew readings.
6. Validate signal behaviour
   • Upstream O2 should switch once in closed loop (petrol). Downstream should be steadier.
   • If signal is stuck high/low, confirm fuel trims and misfire status.
   If trims are extreme, use P0171/P0172 fuel trim flow → or P0300 random misfire flow →
7. After the fix: verify
   • Confirm heater current draw (if available) and that closed loop is achieved quickly after start.