SPN 3698 FMI 3: Meaning and Fix
This fault occurs when the ECM detects voltage above normal on the exhaust system high temperature lamp command circuit, signaling a short to battery or chassis power. Technicians often see this after a forced DPF regeneration or ECM swap, where wiring harness damage or pin corrosion causes the lamp to remain stuck on, misleading the operator about actual exhaust temperature.
Common Symptoms
- Lamp Stays On: Exhaust high temperature lamp remains solidly illuminated even when exhaust temperature is below threshold.
- No Lamp Test: Lamp fails to illuminate during the key-on bulb check sequence, indicating a shorted driver circuit.
- Intermittent Warning: Dash warning flickers or flashes randomly, especially during engine vibration or wet conditions.
- ECM Logged Event: Fault is stored as active or inactive, often accompanied by other voltage-related DPF codes.
Probable Causes
- Short to Battery: Wire between ECM pin and lamp socket chafed against chassis or harness, making contact with 12V/24V.
- ECM Driver Failure: Internal MOSFET in the ECM output stage failed shorted, commanding lamp on continuously.
- Corroded Connector: Moisture ingress at the ECM or dash connector causing pin-to-pin short to power supply line.
- Aftermarket Lamp: Non-OEM lamp with incorrect resistance or missing suppression diode altering voltage feedback to ECM.
Advanced Technical Analysis
The ECM microcontroller monitors the lamp command line via a pull-up resistor and voltage divider network. Under normal ‘off’ state, the line sits at ground. A voltage above 4.5V (12V system) or 9V (24V system) for more than 50ms triggers FMI 3. The firmware applies a debouncing timer to prevent false triggers from transient noise.
Electrical analysis reveals that a short to high typically occurs in the harness section between the ECM output driver and the lamp indicator. The ECM’s internal diagnostic circuit measures voltage at the driver pin; if the lamp circuit is shorted to battery, the driver cannot pull the line low. This is confirmed by measuring 12V at the ECM pin with lamp disconnected.
When FMI 3 is active, the ECM deactivates the lamp driver to protect the output stage and sets a permanent fault. Torque derate is not invoked for this lamp-only command, but the ECM may inhibit DPF regeneration logic if the lamp cannot be reliably controlled, leading to passive regeneration delays and increased soot loading.
Long-term prevention includes inspecting the harness near the exhaust manifold and cab firewall for chafing. In workshops, this code frequently appears after a turbocharger or DPF replacement where harness clips are not re-secured. Using dielectric grease on connectors and verifying lamp resistance (typically 12-18 ohms) prevents recurrence. Always check for aftermarket LED lamps that lack proper load resistors.
Step-by-Step Troubleshooting Guide
- Visual Harness Check: Inspect the lamp command wire from ECM to dash for chafing, burns, or pinched insulation near metal edges.
- Voltage Measurement: Disconnect lamp, measure voltage at ECM pin with ignition on. Greater than 0.5V indicates short to power.
- ECM Output Test: Use a test lamp (21W) connected to ECM pin and ground. If lamp stays on with ignition, driver is shorted.
- Connector Pin Check: Remove ECM connector, check for bent pins, corrosion, or pushed-out terminals in the lamp command circuit.