What does it mean?
P1742 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Automatic Transmission Diagnosis Input Signal Circuit Malfunction .
Symptoms
- MIL (check engine lamp) on, sometimes with the transmission lamp behaving abnormally
- Transmission warning lamp stays on permanently or never lights during the bulb check
- Transmission codes cannot be retrieved, or the module drops off the scan tool
- Default shift strategy with harsh engagement and limited gears
- Faults appear together on both the engine and transmission sides
Likely causes
- Open circuit or broken wire on the diagnosis signal line between the two modules
- Short to ground or short to battery on that signal wire
- Corroded, bent or backed-out pins at either module connector
- Poor module power supply or a corroded common ground affecting the signal reference
- Blown fuse feeding the transmission control side so it cannot output the signal
- Aftermarket wiring, alarm or accessory tapped into the diagnostic line
- Internal module output driver failure, checked after wiring is verified
How to fix it
- Scan both the engine and transmission control sides and record every stored code and freeze frame
- Inspect both module connectors for corrosion, bent terminals and moisture
- Check continuity of the diagnosis signal wire end to end with both connectors unplugged
- Test the same wire for a short to ground and a short to battery voltage
- Verify power and ground at the transmission control side with voltage drop tests
- Disconnect any aftermarket accessory wired into the harness and retest
- Repair the wiring or connector faults first, and replace a module only after the circuit is proven good