SPN 153 FMI 6: Meaning and Fix
SPN 153 FMI 6 indicates the Engine Crankcase Pressure (High Resolution) sensor circuit has detected current above normal or a short to ground. The ECM monitors the sensor’s supply current; when it exceeds a calibrated threshold, typically above 100 mA, this fault is logged. In practice, this code commonly appears after an engine washdown or when wiring chafes against the valve cover, causing a direct ground. Technicians should first inspect the sensor connector for corrosion or bent pins before replacing components.
Common Symptoms
- Check Engine Lamp On: Amber warning lamp illuminates immediately; fault remains active until circuit is repaired and cleared.
- No Performance Loss: Engine typically runs normally; crankcase pressure data is invalid but not used for fueling.
- Erratic Sensor Readings: Live data shows crankcase pressure stuck at 0 kPa or jumping to maximum value sporadically.
- Intermittent Fault Codes: Fault may appear and disappear with vibration; harness movement triggers the short intermittently.
Probable Causes
- Sensor Internal Short: Failed pressure sensor with internal short circuit between supply and ground, drawing excessive current.
- Harness Grounded: Signal or supply wire chafed against engine block or chassis ground, causing direct short.
- Connector Contamination: Oil or moisture in the 3-pin connector creates a conductive path between supply and ground.
- ECM Supply Fault: Internal ECM driver failure supplying 5V reference; excessive current triggers the FMI 6 condition.
Advanced Technical Analysis
The ECM repeatedly applies a 5V reference voltage to the sensor supply pin and measures the current draw. If current exceeds approximately 120 mA for more than 200 ms, the microcontroller sets SPN 153 FMI 6. The signal pin is monitored for voltage, but the primary trigger is high-side current sensing. This debouncing prevents false triggers from transient spikes.
A short-to-ground in the sensor harness creates a low-resistance path (<10 Ω) from the 5V supply to ground. The ECM's current-limiting driver reduces voltage to protect itself, but the fault remains latched. Technicians must measure resistance between sensor supply and ground; values below 100 Ω confirm a short circuit.
When SPN 153 FMI 6 is active, the ECM disables the crankcase pressure sensor input and substitutes a default value of 0 kPa. No torque derate or engine protection is activated because crankcase pressure is not a critical control parameter. However, related diagnostic trouble codes for intake or exhaust systems may be inhibited.
In workshop practice, this fault is often misdiagnosed as a sensor failure when the root cause is a chafed harness near the turbocharger bracket. A visual inspection with a borescope can reveal abrasion. Disconnecting the sensor and measuring resistance at the ECM connector (pins A15 and A16) isolates the harness from the sensor.
Step-by-Step Troubleshooting Guide
- Visual Harness Check: Inspect wiring from sensor to ECM for chafing, especially near valve cover and turbo bracket.
- Connector Resistance Test: Measure resistance between sensor supply and ground; should be >1 MΩ. Below 100 Ω indicates short.
- Sensor Isolation Test: Disconnect sensor; if fault becomes inactive, replace sensor. If fault remains, harness is shorted.
- ECM Pin Check: At ECM connector, measure resistance between pin A15 (supply) and A16 (ground). Repair harness if shorted.