Shift solenoids are activated and deactivated by the vehicle’s computer when it sends an electrical signal through a coil of wire wrapped around a piston mechanism inside the solenoid. If the ECU detects an abnormally high voltage across the F shift solenoid control circuit, it will log a DTC P0999.
How severe is the code?
A P0999 diagnostic code can be very severe, even if the symptoms may not be so bad at first. But they will increase to the point that the vehicle cannot be driven safely.
Symptoms
Symptoms of a P0999 include:
- Shifts are very harsh
- The transmission refuses to upshift
Causes
- Transmission fluid falls below safe operating levels
- Transmission fluid has become too dirty
- Damaged/broken wires
- Broken/corroded connectors
- The shift solenoid has failed
How to diagnose the code?
The diagnostic process for a P0999 OBDII trouble code should begin with checking the transmission fluid level and condition. If it is clean and at the proper level, then the wiring and connectors along the transmission wiring harness should be inspected for signs of damage and corrosion. If there are no issues so far, the F solenoid itself will need to be tested.
What repairs will fix P0999?
- Service the transmission
- Replace or repair damaged wires/connectors
- Replace the shift solenoid