SPN 605 FMI 13: Meaning and Fix
SPN 605 FMI 13 indicates the Refrigerant High Pressure Switch signal is out of calibration, meaning the ECM detects a voltage or frequency outside the expected range for the switch state. This fault commonly appears after a compressor replacement or A/C service if the switch is not properly recalibrated. In practice, technicians often see this code when a new switch is installed without performing the required ECM calibration procedure or when wiring damage alters the signal characteristics, causing the ECM to interpret the normal-pressure state as an invalid value.
Common Symptoms
- A/C Inoperative: Compressor clutch remains disengaged; no cabin cooling despite system charge being correct.
- No Fault Lamp: MIL or A/C warning lamp may not illuminate, as FMI 13 is a calibration fault, not a hard circuit failure.
- Intermittent Clutch: Compressor cycles off randomly at low load; ECM misreads switch state due to offset calibration.
- Stored Fault History: Fault logged as inactive after key cycle; active only when switch signal drifts outside calibrated window.
Probable Causes
- Switch Drift: High pressure switch internal resistance has shifted outside ECM calibration tolerance due to age or contamination.
- ECM Mismatch: ECM software is calibrated for a different switch type (NC vs NO) than physically installed.
- Wiring Resistance: Corroded connector or high-resistance splice adds offset voltage, causing ECM to see out-of-range signal.
- Improper Replacement: Switch replaced without performing factory calibration reset via diagnostic tool, leaving ECM with old reference values.
Advanced Technical Analysis
The ECM monitors the switch signal through a pull-up voltage divider. For FMI 13, the microcontroller compares the raw A/D count against a learned calibration window stored in EEPROM. If the count falls outside the upper or lower threshold by more than 5% of the reference voltage, the fault sets. This logic prevents false compressor disengagement due to minor component variance but requires the switch to be within a tight 0.5–4.5 V window at 25°C.
Electrical noise or contact bounce can trigger a temporary calibration fault if the debouncing timer (typically 100 ms) sees multiple transitions. The ECM’s digital filter averages 16 samples; if the standard deviation exceeds 50 mV, the fault becomes active. This is common on machines with aftermarket A/C compressors that generate higher ripple currents, altering the switch’s ground reference and shifting the perceived calibration point.
Upon detecting FMI 13, the ECM sets the switch state to ‘Pressure normal’ as a safe default, allowing the compressor to run but without active overpressure protection. This can lead to mechanical damage if a real high-pressure event occurs. The ECM does not initiate a torque derate for this fault, but the A/C request signal to the engine control is disabled, reducing parasitic load and preventing potential compressor lockup.
Long-term strategy involves verifying the switch’s resistance curve against the OEM specification (typically 0–10 kΩ for a normally-closed switch). Technicians should use a breakout box to measure the signal voltage at the ECM pin with the engine running. Real-world experience shows that replacing the switch with an OEM part and performing the calibration routine using a factory scan tool (e.g., DDDL or ET) resolves 90% of these faults. The remaining 10% require ECM reflash if the calibration table is corrupted.
Step-by-Step Troubleshooting Guide
- Read Calibration Data: Use J1939 tool to view learned switch range; compare to factory spec in the ECM configuration file.
- Measure Signal Voltage: At ECM connector, key-on engine-off: voltage should be 4.5–5.0 V (NC) or 0.2–0.5 V (NO) at 25°C ambient.
- Perform Switch Test: Apply refrigerant pressure per OEM table; verify switch opens/closes within 10% of rated pressure using multimeter.
- Recalibrate ECM: Execute ‘Calibrate Refrigerant Pressure Switch’ procedure in diagnostic software with ignition on, engine off.