Error Code P1780 is defined as Transmission Control Switch Circuit is Out of Self-Test Range.

This error code is a manufacturer-specific trouble code, meaning it does not apply to all vehicles makes. Rather, it applies only to specific vehicle makes, such as Ford and Toyota. Specification of the definition, troubleshooting, and repairs still vary from one make and model to another.


During KOER (key on, engine off) self-test, the TCS (Transmission Controls Switch) must be cycled. If it’s not cycled, it will cause the code Error Code P1780.

Common Symptoms

As with other error codes, this code activates the Check Engine light and registers the code to the vehicle’s memory system. Other than that, there are usually no drivability or performance issues to the vehicle for this code.

Possible Causes

This code can be caused by more than a couple of factors, such as:

  • TCS not cycled during self-test
  • Defective TCS
  • Open or shorted TCS harness
  • Poor electrical connection in TCS circuit

How to Check

When doing the Key On engine running test for codes, while turning the steering wheel back and forth all the way, try to pump the brake pedal. If you have O/D, turn your O/D button on and off. To these things inside 5 seconds.

If you don’t cycle the O/D switch on and off, you may get the P1780 code.

Simply erase the code and retest.

If the code comes back after conducting the KOER test, then that simply means there’s a short in the O/D switch, or the O/D switch has gone bad.

How to Fix

Perform the KOER test correctly

Replace the TCS

Repair or replace the TCS harness

Repair electrical problem in TCS circuit

In some cases, this code may be caused by left cylinders (4, 5, 6) of the engine being low, which leads to misfires and lower compression readings. If this is the case, then you have a valve problem. Verify this by squirting a few drops of oil into the cylinder and see if compression comes up. If it doesn’t, then the problem is in the bottom end/rings/piston.

9 times out of 10, this problem is simply caused by not performing the test correctly.