SPN 7137 FMI 5: Meaning and Fix
SPN 7137 FMI 5 indicates the Tire 25 pressure sensor circuit has an open circuit or current below normal. This fault commonly appears after a wheel-end repair or tire replacement where the sensor harness is accidentally pinched or left disconnected. Technicians often find this code when the vehicle is brought in for a TPMS warning light, only to discover a corroded pin at the sensor connector.
Common Symptoms
- Warning Lamp: Yellow or red TPMS warning lamp on the dash remains illuminated continuously.
- No Pressure Data: Tire 25 pressure reads 0 kPa or shows dashes on the display.
- Intermittent Signal: Pressure value fluctuates erratically or drops out during vibration on rough roads.
- System Fault Code: J1939 diagnostic tool logs active SPN 7137 FMI 5 with no other tire faults.
Probable Causes
- Open Harness: Broken wire in the sensor harness between the tire pressure module and connector.
- Corroded Connector: Moisture ingress causing pin corrosion at the sensor or chassis-side connector.
- Sensor Failure: Internal open circuit in the tire pressure sensor due to mechanical shock or age.
- Damaged Pigtail: Chafed or cut pigtail wire near the wheel spindle from road debris or tire rub.
Advanced Technical Analysis
The ECM monitors the tire pressure sensor current loop. When the sensor draws less than 4 mA (typical threshold), the ECM sets FMI 5. This happens if the sensor ground or signal wire is open, or if the sensor internal transistor fails open. The ECM debounces the fault for 500 ms before logging it to prevent false triggers from transient noise.
An open circuit can be caused by a break in the shielded twisted pair. The ECM’s pull-up resistor (typically 1 kΩ to 5V) will pull the signal line to 5V when open, but current remains near zero. The diagnostic timer resets only after the circuit current exceeds 6 mA for 1 second continuously.
When this fault is active, the ECM disables the tire pressure monitoring function for that wheel and may inhibit automatic tire inflation systems if equipped. No engine torque derate occurs, but the driver is alerted via a warning lamp. The fault becomes inactive once the circuit current returns to the normal 4-20 mA range.
Long-term prevention includes using dielectric grease on all wheel-end sensor connectors and routing harnesses away from heat sources. Real-world example: a fleet experienced recurrent SPN 7137 FMI 5 on tandem axle trucks; root cause was a factory-installed harness too short, causing tension fractures at the axle bracket. Relocating the bracket and adding service loop eliminated the fault.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Inspect the tire 25 sensor harness for cuts, chafing, or loose connectors at the wheel end.
- Measure Resistance: Disconnect sensor; measure resistance between signal and ground. Open circuit confirms harness or sensor fault.
- Check Supply Voltage: Probe the sensor supply pin (typically 5V) to ground; no voltage indicates open in module supply wire.
- Bypass Test: Jumper a known-good sensor or 1kΩ resistor across signal and ground; if fault clears, replace the sensor.