What Does It Mean?
This is a generic diagnostic trouble code (DTC) for the powertrain and generally applies to OBD-II vehicles. This may include, but is not limited to, vehicles from GMC, Chevrolet, Ford, Buick, Dodge, etc. Although generic, the exact repair steps may vary depending on the year, make, model, and powertrain configuration.
The Transmission Control Module (TCM) monitors all sensors and switches involved in the transmission’s operation. Nowadays, automatic transmissions (also known as A/T) offer more comfort than ever before.
For example, cruise control is sometimes monitored and controlled by the TCM (among other possible modules). In this context, it applies to a normal/performance mode switch, which is activated by the operator to modify shift points and patterns to accommodate generally higher shift points and faster downshift points. The operation of this switch is essential for the performance function, among other systems that may be included. This will vary significantly by manufacturer, so be sure to know WHICH switch applies to your current issue and the specific make and model.
The ECM illuminates the MIL (Malfunction Indicator Lamp) with a P0790 code when it detects a malfunction in the normal/performance switch circuit. Most of the time, this switch is located on or near the gear shift lever. On a column shifter, it may be a button at the end of the lever. On console-style shifters, it may be on the dashboard or console. Since this location varies significantly from vehicle to vehicle, consult your service manual for the exact location.
How Severe Is This DTC?
I would rate this as low severity. That said, you may want to avoid aggressive or hard driving. This could put unnecessary stress on the transmission and its components, so be cautious here.
What Are Some of the Symptoms of the Code?
Symptoms of a P0790 trouble code may include:
Inoperative mode switch
Irregular and/or abnormal switch operation
Inefficient shift pattern
Weak battery
No downshift when torque is needed
What Are Some Common Causes of the Code?
Causes
of this P0790 code may include:
Faulty or damaged switch
Corrosion causing high resistance (e.g., connectors, pins, grounds, etc.)
Wiring issues (e.g., frayed, open, short to power, short to ground, etc.)
Faulty gear shift lever
TCM (Transmission Control Module) issue
Fuse/box problem
What Are the Troubleshooting Steps for P0790?
Basic Step #1
Depending on the tools/references you have, your starting point may vary. That said, if your scanner has monitoring capabilities (DATA STREAM), you may be able to monitor the values and/or operation of your normal/performance switch. If so, toggle the switch on and off to see if your scanner recognizes your inputs. There may be a delay here, so allowing a few seconds is always a good idea when monitoring switches.
Not only that, but if you find the mode switch inoperative according to your scanner, you may be able to jump certain pins at the mode switch connector to rule out the circuit. If the circuit is ruled out this way and the switch is still inoperative, I would move on to testing the switch itself. Obviously, these are general guidelines, but with a moderately capable scan tool, troubleshooting CAN be painless if you know what you’re looking for. Refer to the service manual for specifications/procedures.
Basic Step #2
Test the switch itself, if possible. Most of the time, these switches are only meant to signal the appropriate module(s) (e.g., TCM, BCM (Body Control Module), ECM, etc.) so it can implement modified shift patterns. That said, the majority of those I’ve encountered are on/off style. This means a simple continuity test with an ohmmeter can determine the sensor’s functionality. Now, these sensors are sometimes integrated into the gear shift lever, so be sure to identify the connectors/pins you need to monitor with your multimeter.
NOTE: As with any transmission fault, always ensure fluid levels and quality are adequate and well-maintained.