Transmission Warning Messages and Park light flashing?

2018 GMC TERRAIN
104,000 MILES • 2.0L • 4 CYL • TURBO • 4WD • AUTOMATIC
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MJWOOD05
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Park light was flashing and about 12 hours later, I got a “Service Transmission Now - Unable to Shift Soon” message. I had no issues driving, transmission shifts smoothly. Tan codes and found that there was an “issue” with rear drivers side EPB Actuator. Replaced the actuator, but warnings never went away. Transmission message started becoming more frequent, starting at every 10 minutes, then every 2 minutes.
Checked battery, no issues, transmission fluid is fine, no codes in the TCM, just still a code in the brake system, C028F-04. Started the car and now it says, “Transmission Unable to Shift”.
Any advice on a fix for this? The internet has not been super helpful here, as I’ve seen a lot of these with different “fixes”. Is there a way to flash the system and see if it resets without a scan tool?
Mar 15, 2026 at 10:25 AM
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STRAILER
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Hello,

This is a failsafe, not an actual transmission failure. Without that calibration the module keeps setting C028F-04.

Code C028F-04 Electronic Parking Brake Actuator - Incorrect Signal / Not Calibrated

Sometimes this works on GM vehicles.

1. Turn ignition ON (engine off)
2. Press brake pedal
3. Apply parking brake
4. Release parking brake
5. Repeat 5-6 times
6. Then shut the car off and restart.

If you're lucky, the EPB will relearn. If not you will need to calibrate the actuator, here is a video to show you how.

CAN SCAN - https://youtu.be/u-4syLc-ifQ

You will need a high end scanner FYI. Did you replace the LEFT rear actuator or the RIGHT rear actuator?

The code C028F specifically refers to the left rear actuator circuit. Please go over this and let me know. Ken

Mar 16, 2026 at 9:58 AM
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MJWOOD05
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Ken, thank you so much for the reply. I apologize for the delay, as I've been out of town for a few weeks. I replaced the LEFT (driver's side) parking brake actuator.

I went through all of the steps you mentioned, including utilizing a proper CAN scan tool. The vehicle would not allow the scan tool to do anything with the parking brake actuators, including calibrating. The button on the dash does nothing, and the scan tool gives an error when trying to do anything with the actuators stating, "Control limit exceeded. Incorrect Power Mode. Check Ignition Switch."
Apr 24, 2026 at 7:58 PM
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STRAILER
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No worries, it seems like you have two problems with this car, we will continue with the parking brake actuator issue on this thread, please start new thread for the transmission issue, we only handle one problem per thread, please post your new question here, you must be logged in.

https://www.2carpros.com/questions/new

For the parking brake non operation lets check the ABS fuses both the 30 amp and 60 amp fuses in the penal under the hood, this video can help you.

https://www.youtube.com/watch?v=G-xNK79Wa-w

Let us know what you find.


Apr 25, 2026 at 8:15 AM