SPN 1659 FMI 31: Meaning and Fix
SPN 1659 FMI 31 indicates the engine coolant system thermostat 1 has a condition exists fault, meaning the ECM detects the electronic thermostat is not operating within its expected range. This code commonly appears after a forced DPF regeneration when the thermostat fails to close properly, causing erratic coolant temperature readings and potential engine overheating.
Common Symptoms
- Erratic coolant temp: Coolant temperature fluctuates unpredictably on the dash gauge and J1939 datalink.
- Engine overheating: Engine runs hotter than normal, especially under load, due to stuck-open thermostat.
- Reduced fuel economy: Engine fails to reach optimal operating temperature, causing increased fuel consumption.
- Active derate active: ECM initiates a torque derate to protect the engine from thermal stress.
Probable Causes
- Stuck thermostat: Electronic thermostat mechanically stuck open or closed due to debris or wear.
- Wiring fault: Open or short circuit in the thermostat signal or power supply harness.
- ECM software glitch: Outdated ECM calibration causing incorrect thermostat position detection.
- Coolant contamination: Contaminated coolant with debris blocking the thermostat valve movement.
Advanced Technical Analysis
The ECM monitors the electronic thermostat position via a PWM signal from a Hall-effect sensor. When the duty cycle deviates from the expected 5-95% range for more than 5 seconds, the internal diagnostic sets FMI 31. The microcontroller compares actual position to a modeled position based on coolant temperature and engine load.
Electrical analysis shows that a failed thermostat often presents with a stuck signal at 0% or 100% duty cycle. The ECM includes a debouncing timer of 2.5 seconds to filter transient noise from the J1939 datalink. If the condition persists, the fault is latched and the amber warning lamp is illuminated.
Upon activation of this fault, the ECM enters a safety fallback mode: it forces the thermostat to a default open position by applying full PWM to the heater element. This prevents overheating but causes prolonged warm-up. The ECM also reduces maximum engine torque by 25% to limit thermal stress.
Long-term diagnosis requires monitoring the thermostat response during a controlled cool-down cycle. Technicians frequently encounter this fault after replacing the ECM without performing a thermostat calibration reset. Using a diagnostic tool to command the thermostat open/closed while observing coolant temperature change is the most reliable verification method.
Step-by-Step Troubleshooting Guide
- Scan for codes: Read all active and inactive SPNs; check for related coolant temperature sensor faults.
- Inspect wiring: Visually check the thermostat connector and harness for chafing, corrosion, or loose pins.
- Test thermostat: Use a diagnostic tool to command the thermostat open/closed while monitoring PWM feedback.
- Check coolant: Verify coolant level and condition; replace if contaminated with debris or incorrect mixture.