SPN 2623 FMI 16: Meaning and Fix
SPN 2623 FMI 16 indicates that the secondary output channel of the redundant accelerator pedal position sensor #1 has reported a valid but excessively high percentage signal, exceeding the calibrated upper threshold defined by SAE J1939 and OEM ECM parameters. This fault commonly appears after accelerator pedal assembly replacement when the new sensor’s voltage divider ratio is mismatched to ECM expectations, causing Channel 2 to read disproportionately high relative to Channel 1, triggering a plausibility fault and potential torque limitation.
Common Symptoms
- Reduced Engine Power: ECM applies torque derate strategy when Channel 2 signal persistently exceeds normal range, limiting maximum available engine output.
- Sluggish Throttle Response: Transmission and ASR algorithms receive corrupted anticipatory pedal data, causing delayed gear selection and erratic acceleration behavior.
- Active Fault Lamp: Amber MIL or engine warning lamp activates immediately upon fault confirmation after debounce timer expires within ECM logic.
- Cruise Control Inhibition: ECM disables cruise control functionality as a safety measure when redundant pedal sensor channels fail plausibility cross-check validation.
Probable Causes
- Pedal Sensor Miscalibration: Replacement accelerator pedal assemblies with incorrect output voltage curves cause Channel 2 to exceed programmed ECM acceptance thresholds.
- Wiring Harness Damage: Chafed or shorted signal wires near the pedal bracket elevate Channel 2 voltage artificially, simulating an above-normal position reading.
- 5V Reference Contamination: Shared 5-volt reference line affected by sensor supply leakage inflates Channel 2 output beyond normal operating percentage range.
- ECM Software Misconfiguration: Incorrect parameter sets after ECM reflashing may shift upper plausibility limits, flagging valid Channel 2 signals as out-of-range.
Advanced Technical Analysis
The ECM microcontroller continuously samples both Channel 1 and Channel 2 outputs of the redundant accelerator pedal sensor at high frequency, typically every 10 milliseconds per Bosch EDC17 architecture documentation. SPN 2623 FMI 16 triggers when Channel 2 percentage exceeds the OEM-defined upper plausibility band, often set between 95–98% of maximum pedal travel, while Channel 1 remains within normal range. This inter-channel divergence is interpreted as a sensor fault rather than actual pedal position.
Electrically, the redundant sensor design uses two separate resistive tracks within the same housing, each fed by the 5V reference but producing offset voltage curves. Channel 2 typically outputs a declining voltage as pedal depression increases, inverse to Channel 1. A short to supply voltage or degraded ground reference along Channel 2’s dedicated signal wire forces the ECM-calculated percentage above threshold. Bosch and MAN factory wiring diagnostics recommend measuring Channel 2 signal voltage at the ECM connector pin under static and dynamic conditions before replacing the pedal assembly.
Upon fault confirmation, the EEC-15 controller activates a graduated fallback strategy consistent with SAE J1939 FMI 16 severity classification. Engine torque output is typically limited to 60–70% of rated capacity, and the transmission control unit receives a frozen or default pedal demand value via the CAN bus. Mercedes-Benz and Deutz service documentation specifies that the derate remains active until the fault clears across a minimum of three consecutive key cycles, preventing intermittent sensor faults from masking underlying wiring problems.
In workshop practice, this fault frequently reappears within weeks of pedal sensor replacement when technicians source non-OEM compatible assemblies with differing output ranges. Deutz and MAN field service bulletins recommend verifying Channel 2 voltage sweep from 0.5V to 4.5V across full pedal travel using a calibrated oscilloscope before closing the repair. Long-term prevention involves applying dielectric grease to the pedal harness connector and securing the harness with OEM routing clips to prevent vibration-induced insulation wear on heavy construction equipment.
Step-by-Step Troubleshooting Guide
- Channel Voltage Measurement: Measure Channel 2 signal voltage at ECM connector with pedal released and fully depressed; compare against OEM-specified sweep range.
- Harness Continuity Inspection: Perform resistance and insulation tests on Channel 2 signal wire from pedal connector to ECM pin, checking for shorts to supply.
- Cross-Channel Plausibility Check: Compare Channel 1 versus Channel 2 percentage readings live via diagnostic tool; divergence exceeding 5% confirms sensor or wiring fault.
- ECM Parameter Verification: Confirm ECM calibration dataset matches pedal assembly part number using manufacturer diagnostic software to rule out software misconfiguration.