What does it mean?
P1378 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: FICM Supply Voltage Low Input .
Symptoms
- MIL on, hard starting or very long cranking when cold
- No-start once the module voltage falls far enough
- Rough running, misfire and white smoke after a cold start
- Loss of power and stalling under load
- Battery or charging warning may appear at the same time
Likely causes
- Weak, discharged or mismatched batteries
- Corroded or loose battery cable ends and engine ground straps
- Failing internal power supply inside the module itself
- Blown fuse or a faulty relay in the module feed circuit
- Charging system undercharging, alternator output low
- Chafed or high resistance harness section in the module power feed
- Corroded module connector with water intrusion
How to fix it
- Load test both batteries and clean and tighten every cable end before anything else
- Measure charging voltage at idle and at raised engine speed
- Read the module supply voltage in live data while cranking and while running, and compare it with battery voltage
- Perform voltage drop tests on the module power feed and its ground path
- Inspect the module connector for corrosion, moisture and spread terminals
- Check the fuse and relay feeding the module
- Replace or rebuild the module only after the batteries, cables and charging output are confirmed good