What does it mean?
P1804 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Shift Fork Position Sensor 1 and 2 System (Power supply voltage low range out) .
Symptoms
- MIL (check engine lamp) is on and the transmission is in limp mode
- The vehicle will not shift, or holds a single gear only
- Harsh or refused engagement, with clutch actuation errors on automated units
- On some vehicles the car will not move at all
- The gear display shows dashes or the wrong gear
- The fault can appear on a cold start and clear once the engine bay warms
Likely causes
- Open circuit or high resistance in the sensor supply feed from the control unit
- Short to ground on the shared supply line, which drags both sensors down together
- One of the two position sensors failed internally and loading the shared supply
- Corroded or water-damaged connector at the transmission, especially where it is exposed underneath
- Harness chafed inside the transmission or at the case pass-through
- Weak battery, poor charging or a corroded control unit ground
- Failed supply regulator inside the control unit
How to fix it
- Test the battery and charging voltage, then clean and tighten the control unit ground points
- Measure the supply voltage right at each sensor connector with the ignition on and compare with specification
- Unplug the sensors one at a time and watch whether the supply voltage recovers, which identifies a shorted sensor
- With everything disconnected, check the supply wire for continuity and for a short to ground along the whole run
- Inspect the transmission connector for water, fluid and corroded pins, and repair or reseal it
- Replace the faulty sensor or repair the harness, then run the shift fork position learn procedure and road test