SPN 1328 FMI 4: Meaning and Fix
This fault indicates the Engine Control Module (ECM) has detected a voltage below the normal operating threshold on the Cylinder 6 misfire rate signal circuit. In practice, this code commonly appears after a forced DPF regeneration or following replacement of the ECM, when the sensor harness is inadvertently pinched against the valve cover. The signal remains stuck low, preventing accurate misfire percentage calculation per SAE J1939-71.
Common Symptoms
- Rough Idle: Cylinder 6 misfire causes uneven crankshaft acceleration at idle, noticeable as vibration in the cab.
- Reduced Power: ECM initiates torque derate to protect aftertreatment, limiting engine output to 50-60%.
- MIL Illuminated: Red stop lamp and amber warning lamp activate per J1939 diagnostic message 1.
- Excessive Smoke: Unburned fuel from Cylinder 6 exits as white smoke during cold operation.
Probable Causes
- Shorted Sensor Wire: Insulation rubbed through on engine bracket, shorting signal to ground.
- Failed Ionization Sensor: Internal short in the Cylinder 6 glow plug or combustion pressure sensor.
- ECM Internal Fault: Pull-up resistor failure in the ECM input circuit for Cylinder 6.
- Corroded Connector: Pin 12 at the ECM J2 connector exhibits green corrosion from moisture ingress.
Advanced Technical Analysis
The ECM monitors the Cylinder 6 misfire rate signal via a dedicated analog input with a 5V pull-up resistor. When the signal voltage drops below 0.5V for 200 consecutive engine cycles, the ECM sets FMI 4. This threshold is derived from Bosch EDC17 calibration tables, ensuring noise immunity from ignition interference.
The debouncing timer prevents false triggering from transient voltage spikes. A short-to-low condition must persist through 200 cycles (approximately 2 seconds at 600 RPM) before the fault is logged. This aligns with Deutz common rail service manual guidelines for combustion feedback diagnostics.
Upon activation, the ECM transitions to a safety fallback mode: it disables fuel injection to Cylinder 6 and reduces overall torque by 40%. This protects the DPF from excessive soot loading. The derate remains active until the fault clears and a successful key cycle completes, per MAN TGX workshop procedures.
Long-term diagnosis should include measuring the sensor output at the ECM pin with a digital multimeter. Technicians frequently find that after replacing the injector on Cylinder 6, the harness was not properly secured, causing a chafed wire. Using Mercedes-Benz Xentry, the signal voltage should be verified at 4.8V with engine off and 1.2V during cranking.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check Cylinder 6 harness for chafing against the exhaust manifold or valve cover.
- Measure Voltage: Backprobe ECM J2 pin 12; should read 4.8V KOEO. Below 0.5V indicates short.
- Isolate Sensor: Disconnect Cylinder 6 sensor; if voltage rises to 5V, sensor is shorted internally.
- Continuity Check: Measure resistance from sensor signal pin to ground; should be infinite.