SPN 1547 FMI 3: Meaning and Fix
The A/C evaporator temperature sensor signal voltage exceeds the normal range, typically above 4.8 V. This fault often appears after a wiring repair near the evaporator housing or following a refrigerant system service where the connector was damaged. Technicians frequently encounter this code when the sensor harness rubs against the HVAC box, causing a short to battery voltage.
Common Symptoms
- No Cabin Cooling: Compressor clutch remains disengaged due to invalid evaporator temperature data from ECM.
- Erratic Blower Speed: Climate control module fails to regulate fan speed when sensor signal is out of range.
- High Side Pressure Spike: Without proper evaporator temp feedback, expansion valve may overfeed liquid refrigerant.
- Dashboard Warning Light: Amber A/C fault lamp illuminates on the instrument cluster, often with a diagnostic code.
Probable Causes
- Sensor Supply Short: 5V reference wire chafed against chassis ground or shorted to 24V power source.
- Connector Corrosion: Moisture ingress at the two-pin connector causes increased resistance or intermittent short.
- Failed NTC Sensor: Internal thermistor failure results in an open circuit, pulling signal voltage high.
- ECM Pull-Up Resistor: Internal ECM pull-up resistor failure can falsely indicate a high voltage condition.
Advanced Technical Analysis
The ECM monitors the evaporator temperature sensor via a 5V pull-up resistor network. Under normal operation, the NTC thermistor voltage decreases as temperature rises. When the signal line exceeds 4.8V for more than 200 ms, the J1939 diagnostic sets FMI 3. The debounce timer prevents false triggering from transient spikes during compressor engagement.
Electrically, a short to high source (battery or 5V reference) bypasses the thermistor, forcing the ADC input to near 5V. The ECM compares this value against a calibrated upper threshold of 4.85V. If the reading stays above threshold for the debounce period, the fault becomes active. A short to ground would produce FMI 4, not FMI 3.
Upon detecting FMI 3, the ECM defaults the evaporator temperature to a fixed value (typically 10°C) to allow limited A/C operation. Some OEMs disable the compressor entirely to prevent evaporator icing. Torque derate is not applied, but cabin comfort is severely degraded. The system remains in this safe mode until the fault is cleared.
Long-term prevention requires inspecting the harness routing near the HVAC box for chafing. In real workshops, this code commonly appears after a cabin filter replacement where the sensor wire was pinched. Always perform a wiggle test after repair. If the sensor itself is faulty, replace with an OEM part; aftermarket sensors often have different resistance curves causing recurring faults.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check wiring from sensor connector to ECM for chafing, cuts, or pinching near HVAC housing.
- Voltage Measurement: With key on, measure signal pin voltage; should be below 4.8V. If above, disconnect sensor.
- Sensor Resistance Test: Measure sensor resistance at 25°C: typical NTC is 10kΩ. Open circuit indicates failure.
- Connector Pin Check: Inspect for bent pins or corrosion. Clean with contact cleaner and apply dielectric grease.