What does it mean?
P1640 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Output Driver Module 'A' Circuit Malfunction .
Symptoms
- MIL (check engine lamp) on together with circuit codes for the components in that driver group
- One or more outputs dead: a relay that will not click, a solenoid that will not move, a lamp that will not light
- A cooling fan, purge valve or similar component stuck on or never operating
- Rough running, poor emissions or overheating depending on which output is lost
- A fuse that keeps blowing after replacement
- Limp mode and reduced power in some cases
Likely causes
- A shorted actuator on one of that group's outputs: a relay coil, solenoid, valve winding or lamp
- Output wiring chafed and shorted to ground or to battery positive
- Corroded or water-damaged connector on one of the driven circuits
- Blown fuse or a fault in the supply feeding the driven components
- Overvoltage event, such as jump starting, damaging the driver stage
- Poor module ground shifting the driver reference
- Internal failure of the output driver stage inside the module
How to fix it
- List every stored code; the group of affected components identifies which driver circuit is at fault
- Use scan tool output tests to command each component in the group and note which ones respond
- Measure the winding resistance of each actuator against specification to find a shorted load
- Disconnect the suspect actuators one at a time and see whether the driver code stops setting
- With the actuators unplugged, check the output wiring for shorts to ground and to battery positive
- Voltage-drop test the module grounds and confirm the supply feeding the driven circuits
- Repair the shorted component or wiring first; replace and program the module only if the driver still fails with every load disconnected