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Error Code P1790 is defined as Fault Immediate After Shift.
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 Acura, Chrysler/Dodge/Jeep, Citroen/Peugeot, Daewoo, Ford, GM, Honda, Isuzu, Jaguar, Kia, Mercedes, Opel, Toyota, Volkswagen/Audi, and Volvo. Specification on the definition, troubleshooting, and repairs still vary from one make and model to another.
Error Code P1790 is stored when the Gear Ratio DTC has been immediately detected after shift. The existence of this DTC indicates a mechanical or hydraulic (not electrical) problem.
It is important to note that not all mechanical problems necessarily lead to DTC code. If this error code exists, the diagnosis must be based on the associated DTC, and only the mechanical causes of the problem must be considered.
As with other error codes, this code activates the Check Engine light and registers the code to the system. The vehicle may also jump from time to time during braking, and harsh downshift when somewhere around 30MPH while slowing down.
- Low fluid due to leak
- Bad transmission pump, leaking reverse piston, or accumulator cover
How to Check
In many cases, this code comes with another set of error codes.
If the code is characterized by a harsh downshift symptom during 30MPH slowing down, then there’s a good chance you don’t need any major repairs.
Harsh downshift usually comes from an underdrive clutch volume index being too high, as it is applying the UD clutch too hard at 2-3 downshifts. This usually happens when the fluid is too low due to leak. Obviously, you must check the fluid level with the transmission hot, engine idling, and transmission in park.
If the fluid is full, then you may have a bad pump problem in the transmission, leaking low reverse piston or accumulator.
How to Fix
Add transmission fluid.
For a bad pump problem, leaking low reverse piston or accumulator, there is a software update for the transmission controller to fix your exact problems. Obviously, that needs to be done by the dealer before any internal work in the trans must be done, especially if you don’t have any visible or drivability symptoms.
Whatever the cause of the problem may be, harsh downshifting can be repaired (at least for some time) by doing a quick learn procedure. This is done by using a scan tool to set the volume indexes back to the baseline, to let them completely relearn. This test only takes a few minutes.