SPN 6918 FMI 21: Meaning and Fix
SPN 6918 FMI 21 indicates the SCR system cleaning inhibit switch signal has drifted below the valid range, locking out active cleaning events. In practice, this fault often appears after an ECM replacement or wiring repair where the switch circuit remains open. The system will not initiate or will prematurely abort a cleaning cycle, risking eventual catalyst fouling.
Common Symptoms
- Cleaning Inhibited: SCR system cleaning will not start or aborts immediately after beginning, as per SAE J1939-71.
- Dashboard Warning: Amber MIL and SCR system warning lamp illuminate, indicating a non-critical but persistent fault.
- Torque Derate: Engine torque may be gradually reduced after extended operation without cleaning events.
- No Regeneration: Active or stationary regeneration is blocked, preventing soot and ash removal from the DPF.
Probable Causes
- Faulty Inhibit Switch: Switch stuck in closed position or internal resistance too low, pulling signal voltage down.
- Open Ground Circuit: Chassis ground for the switch circuit has high resistance, causing voltage drift below threshold.
- ECM Input Damage: Internal pull-up resistor failure in ECM allows signal to float low when switch opens.
- Wiring Harness Short: Signal wire shorted to ground in the harness, commonly near the SCR module connector.
Advanced Technical Analysis
The ECM monitors the inhibit switch input on a dedicated digital input pin with a 5V pull-up. When the switch is closed (inhibited), the voltage is pulled low (<1.0V). FMI 21 triggers when the voltage drifts below 0.5V for more than 200 ms, indicating a ground short or stuck switch. This prevents false cleaning events during parked regeneration.
A debouncing timer of 150 ms is implemented to filter noise. If the low state persists beyond this timer, the ECM sets the fault and locks the cleaning inhibit flag in non-volatile memory. The flag remains set until a valid high state (switch open) is detected for 5 consecutive seconds, per Bosch MD1 CP3 calibration guidelines.
Once inhibited, the SCR system cleaning event is blocked. The ECM reduces engine torque by 25% after 10 hours of operation without a cleaning cycle, as per Deutz factory fallback maps. This protects the SCR catalyst from excessive soot loading. The driver will notice gradual power loss, especially during high-load operation.
Technicians frequently encounter this fault after replacing the ECM without transferring the inhibit switch calibration. Always verify the switch resistance between 0.5 and 1.5 ohms when closed. In MAN trucks, a common cause is corrosion in the 12-pin SCR connector near the frame rail. Use a breakout box to measure pin voltage before condemning the ECM.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check the inhibit switch and harness for damage, corrosion, or loose connections near the SCR module.
- Voltage Measurement: At ECM pin, measure voltage with switch open: expect 4.5-5.0V; with closed: <0.5V.
- Switch Resistance Test: Disconnect switch; measure resistance closed: 0.5-1.5 ohms; open: >10k ohms.
- Clear and Verify: Clear fault, cycle ignition, and perform a manual cleaning event to confirm inhibition is removed.