B2509

Step 1: Perform visual inspection of rear fog lamp switch and associated connectors for physical damage, corrosion, bent pins, or moisture intrusion. Check connector locking mechanisms are secure.

Step 2: Using a digital multimeter, test for proper voltage supply at the rear fog lamp switch connector (typically 12V reference voltage when ignition is on). Verify ground circuit continuity to chassis ground.

Step 3: With switch disconnected, measure resistance across switch terminals in both ON and OFF positions. Compare readings to manufacturer specifications (typically open circuit when off, less than 5 ohms when on). Actuate switch multiple times while monitoring resistance for intermittent failures.

Step 4: Test circuit continuity between switch connector and BCM connector using multimeter or wiring diagram. Check for shorts to ground or power by measuring resistance to ground and battery voltage on each circuit wire with all components disconnected.

Step 5: If all circuits test normal, perform BCM input monitoring using live data stream to verify BCM recognizes switch actuation. Replace switch if BCM does not register input changes when physically actuating switch with known-good power and ground.

Step 6: Clear codes, reconnect all components, and perform functional test of rear fog lamp operation. Road test vehicle and verify code does not return after multiple switch cycles and various environmental conditions.