Error Code P0781 is defined as 1-2 Shift Malfunction. This is a generic trouble code, meaning it applies to all vehicles with the OBD-II system, especially those made since 1996 up to present and equipped with automatic transmission. Specifications on the definition and steps for repairs may vary depending on the model and make of the vehicle.
For vehicles equipped with automatic transmission, the 1-2 shift solenoid works by actuating the hydraulic circuits, activating the bands or clutches that changes the gears inside the transmission. The shift solenoids are controlled by either the TCM (Transmission Control Module) or PCM (Powertrain Control Module, also known as ECM or Engine Control Unit in other vehicle makes). Depending on the application, this control circuit may be power or ground side controlled.
Other vehicles conduct self-tests on the circuit even when the gear is not in use. The 1-2 shift solenoid can be an on/off solenoid, or a duty cycle controlled. The PCM or TCM searches for an inductive kick to determine whether the solenoid is working properly even when not in use. The brief activation can be too fast for the solenoid to cause a shift in the transmission. When the solenoid is activated and turned off, the magnetic field will collapse and send a voltage that is slightly higher than usual to the PCM. The PCM uses this voltage to determine if the solenoid is working properly.
Other related error codes include:
This code may activate the Check Engine light and possibly shut the transmission controls down altogether, resulting in a “failsafe” mode. This means it will only allow one default gear, depending on the state of the vehicle with all solenoids off. Another common symptom includes:
- Vehicle unable to shift from 1st to 2nd gear
- Failure of 1-2 shift solenoid
- Pinched wires resulting in an electrical short to power
- Pinched wires resulting in an electrical short to ground
- Open in power or ground circuit wiring (loose or disconnected connections)
- Internal failure in the TCM or PCM
How to Check
The first step is to disconnect the PCM/TCM’s wiring harness. Depending on the application, use the electrical diagram as a reference. Make sure to also check for excessive resistance or an open in the circuit.
Use the DVOM (digital volt ohm meter) set at ohms and check for the resistance in the 1-2 solenoid power and ground to find out if there’s an open or excessive resistance. Inspect the connectors and wires, make sure there are no disconnected or loose wires on the harness connectors.
Check the transmission fluid:
If you find no electrical issues, then the problem could be in the transmission fluid. Make sure the transmission fluid is not dirty or contaminated, doesn’t have burnt smell, and at the correct level. Also, note that too much fluid is just as bad as too little. So follow the specifications on fluid replacement. You can also take the vehicle to the dealer for further diagnosis and repair.
Some newer vehicle models don’t have transmission dipsticks. Thus, it may be necessary to get a sample transmission fluid from the fluid cooler lines. Check the manual to find the cooler lines. Be very careful not to get any dirt into the fluid cooling lines.
If the transmission fluid appears dark or has an abnormal smell, then replace it. Remember that engine overheating can degrade transmission fluid, so make sure the vehicle’s engine has not experienced overheat problem in the recent past.
If the problem is short to power:
Disconnect the positive cable of the battery and wiring harness connector at the transmission. Use the DVOM to check for continuity between the power source and solenoid circuit wiring in KOEO (key on/engine off) position, and the key off position.
If the problem is short to ground:
Disconnect the positive cable of the battery and wiring harness connector at the transmission. Use the DVOM to check for the continuity between the power source of the 1-2 solenoid and a known good ground.
If the problem is caused by failed PCM:
Use an advanced scan tool that includes controls for the solenoid. Activate the 1-2 shift solenoid manually. For some vehicle, you may need to activate the transmission control relay manually if there is no power present at the transmission wiring harness connector.
If you don’t have a scan tool, then monitor the 1-2 solenoid circuit to find out whether the TCM or PCM is sending a power or ground signal to the 1-2 solenoid during the vehicle’s operation.
How to Fix
In most cases, the Error Code P0781 is a matter of maintenance than repair. Often, simply topping off or replacing the transmission fluid will correct the problem.
Repair or replacement of wiring problems associated with continuity.
Mechanics may assume the worst and blame the shift solenoid or the PCM when the problem is dirty transmission fluid or low fluid level. Shift solenoids are seldom the problem, while the PCM is hardly an issue in most cases.
Trouble shifting may not be a dangerous issue when driving around. But if you’re attempting to pass on the Interstate, then this could be a huge problem. Thus, you need to get this code checked at the earliest convenience. The last thing you want when driving at high speeds are surprises from your vehicle’s performance.