SPN 6780 FMI 20: Meaning and Fix
SPN 6780 FMI 20 signals that the aftertreatment 1 particulate sensor temperature reading has drifted above a plausibility threshold, often due to sensor contamination or wiring resistance. This code commonly appears after a forced DPF regeneration when the sensor is exposed to excessive heat or soot buildup, causing inaccurate temperature data that the ECM interprets as a high drift.
Common Symptoms
- Reduced Engine Power: ECM may derate engine torque to protect aftertreatment components when sensor temperature data is unreliable.
- DPF Regeneration Inhibited: Active regeneration may be disabled because the sensor cannot provide accurate temperature feedback for safe regeneration.
- MIL Illumination: Malfunction indicator lamp (MIL) or CEL turns on to alert the driver of an emissions-related fault.
- Excessive Soot Accumulation: Without correct sensor data, regeneration timing is altered, leading to faster soot loading and increased backpressure.
Probable Causes
- Sensor Contamination: Soot, ash, or oil residue on the sensor element changes its thermal response, causing elevated readings.
- Wiring Resistance: High resistance in sensor signal or ground circuits due to corrosion or damaged connectors alters voltage levels.
- Faulty Sensor Element: Internal degradation of the thermistor or RTD element leads to drift outside the expected temperature range.
- ECM Software Anomaly: Incorrect calibration or software bug in the aftertreatment control module can misinterpret valid sensor data as drifted high.
Advanced Technical Analysis
The ECM continuously monitors the particulate sensor temperature via a 5V reference circuit, comparing the voltage drop across the sensor to a calibrated temperature curve. When the signal voltage corresponds to a temperature above a programmed plausibility limit (e.g., >900°C), the diagnostic is set. The ECM uses a debouncing timer to confirm the fault, typically requiring the condition to persist for several seconds to avoid false triggers from transient thermal spikes during regeneration.
Electrical analysis often reveals that the sensor signal circuit is pulled high due to an open ground or short to power. Using a digital multimeter, technicians can measure the sensor resistance at known temperatures and compare it to factory specifications. For example, a typical particulate sensor may have a resistance of 1000 ohms at 25°C, but if it reads 5000 ohms, the ECM interprets this as a high temperature, triggering FMI 20.
When this fault is active, the ECM enters a safe fallback mode, disabling active DPF regeneration and potentially derating engine torque by up to 25% to prevent uncontrolled exothermic reactions. The aftertreatment system may also command a forced regeneration inhibit, which can lead to increased soot loading and eventual passive regeneration attempts, further stressing the sensor.
Long-term diagnostics should include a thorough inspection of the sensor harness for chafing or corrosion, especially near the exhaust manifold where heat and vibration are high. In workshop practice, this code often appears after a sensor replacement if the connector is not fully seated, or after a DPF cleaning when residual ash contaminates the sensor. Regular sensor cleaning with compressed air and checking for proper torque during installation can prevent recurrence.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Inspect sensor and harness for damage, corrosion, or loose connections. Pay special attention to the exhaust area.
- Measure Sensor Resistance: Disconnect the sensor and measure its resistance at ambient temperature. Compare to factory spec; typical is around 1000 ohms at 25°C.
- Check Wiring Continuity: Using a multimeter, verify continuity and isolation of signal, ground, and reference wires. Look for shorts to power or ground.
- Monitor Live Data: Use diagnostic tool to view particulate sensor temperature. Compare with exhaust gas temperature sensors; if drift is present, replace sensor.