What does it mean?
P1787 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: A/T FFS Signal Circuit Malfunction .
Symptoms
- MIL (check engine lamp) is on and additional transmission codes are usually stored with it
- The transmission shifts harshly or at the wrong points
- Limp mode with a single gear available
- Delayed engagement when a gear is selected
- Reduced drivability and higher fuel consumption
- Symptoms can come and go, especially over rough roads
Likely causes
- Open circuit or a high-resistance joint in the signal wire
- Short to ground on the signal line where the harness passes a bracket or bulkhead
- Short to battery voltage from insulation damage next to a power feed
- Corroded or loose connector at either end of the circuit, especially in a damp location
- The unit generating the signal is faulty or not powered, so nothing arrives at the transmission module
- Weak battery, poor grounds or an unstable charging voltage disturbing the signal level
- Failed input circuit inside the transmission control module
How to fix it
- Record all stored codes and freeze frames, and note which conditions bring the fault back
- Back-probe the signal at the transmission module connector and observe it during a road test
- Disconnect both ends and check the wire for continuity, for a short to ground and for a short to battery
- Inspect connectors for corrosion, moisture and loose terminal tension; wiggle-test while watching live data
- Check the supply and ground of the unit that generates the signal before suspecting it
- Repair the wiring or connector; replace a control unit only after the circuit and its source are proven good