SPN 111 FMI 20: Meaning and Fix
This fault indicates the Engine Coolant Level 1 sensor signal has drifted above the valid range, often due to sensor aging or contamination. Technicians frequently encounter this after a coolant system flush where air bubbles or debris cause erratic readings, leading to false high-level signals that trigger the ECM’s plausibility check.
Common Symptoms
- False High Level: ECM reports coolant level above 100% despite normal or low actual coolant volume.
- Intermittent Warning: Dash coolant warning light flickers on and off during engine operation, especially on inclines.
- No Derate Yet: Engine runs normally initially; torque reduction activates only after fault persists for 30 seconds.
- Logging Events: Multiple occurrences logged in ECM memory, often correlating with thermal expansion cycles.
Probable Causes
- Sensor Drift: Capacitive or resistive coolant level sensor output slowly increases due to internal degradation.
- Signal Short: Sensor signal wire shorted to battery voltage or a reference voltage, pulling the reading high.
- Contaminated Coolant: High conductivity or air bubbles in coolant alter the dielectric constant, skewing sensor output.
- ECM Reference Fault: Internal ECM 5V reference supply drifting high affects all analog inputs including coolant level.
Advanced Technical Analysis
The ECM monitors the coolant level sensor via a 5V pull-up resistor and ADC input. A valid range of 0.5V to 4.5V corresponds to 0-100% level. FMI 20 triggers when the ADC reading exceeds 4.55V for more than 10 consecutive samples, indicating a drift above the maximum calibrated threshold.
Electrical analysis reveals that a short to 5V or battery positive (24V nominal) causes immediate high drift. The ECM’s debouncing timer filters transient spikes, but persistent high readings exceeding 500ms latch the fault. Real-world cases show intermittent shorts in the engine harness near the expansion tank connector.
Upon activation, the ECM sets the coolant level to a default safe value of 80% and enables a progressive torque derate of 25% after 30 seconds. The engine may also activate the cooling fan continuously to prevent overheating, as the system assumes possible coolant loss despite the high signal.
Long-term diagnosis requires monitoring sensor voltage with a breakout box. In practice, this code often appears after a coolant change where air is trapped, causing the sensor to read high until the system bleeds. MAN and Deutz manuals recommend verifying sensor ground integrity and checking for coolant additive buildup on the sensor probe.
Step-by-Step Troubleshooting Guide
- Read Live Data: Use diagnostic tool to monitor coolant level voltage; compare to actual coolant level in expansion tank.
- Inspect Connector: Check for bent pins, corrosion, or moisture at the sensor connector and ECM harness side.
- Measure Sensor: Disconnect sensor; measure resistance between signal and ground. Should be open circuit (infinite).
- Test Wiring: With key off, check for voltage on signal wire. Any voltage >0.5V indicates a short to power.