GMC Terrain Won't Start: Multiple Transmission Issues

Back in 2014, I dealt with nearly identical symptoms on my GMC Acadia (2010 model, 3.6L gasoline). As someone with moderate DIY experience, these multiple symptoms initially had me stumped. The root cause turned out to be a defective transmission control unit, which was affecting the gear shifter position sensor and causing communication issues across multiple systems. The automatic transmission would show similar symptoms to what you describe, inconsistent gear changes and position readings. The repair required professional diagnosis since the transmission issues were tied to electronic components rather than mechanical wear. Total repair cost was 1000 Euro, covering the control unit replacement and programming. Before jumping to conclusions though, could you share: Current mileage on your Terrain, Any recent transmission maintenance history, Specific error codes being stored, Whether the gear shifter moves smoothly through all positions These details would help determine if your symptoms match what I experienced with the control unit failure. The good news is that my transmission repair resolved all the related issues, including the starting problems and error codes.

Last service was done at 120070 KM about 2 weeks ago. The service included regular maintenance but no transmission work. The error codes stored are P0700 and P0706. I have checked and the shift lock seems normal when moving between gears, though there is a slight delay when shifting from park to drive. The automatic transmission does occasionally slip between 2nd and 3rd gear. The transmission sensor readings from my OBD scanner suggest inconsistent position detection, particularly in park position. When attempting to diagnose the transmission issues myself, I noticed the symptoms worsen after the engine reaches normal operating temperature. I am considering having a professional transmission repair shop look at it, given the complexity of these transmission issues. Would appreciate knowing if anyone else has dealt with similar error codes on their Terrain.

Thanks for providing those details about your GMC Terrain. The symptoms and error codes align perfectly with what I experienced. P0700 and P0706 are definitely transmission sensor related, specifically pointing to gear linkage and position sensor problems. In my case, the transmission issues also worsened with engine temperature, just like yours. The delay when shifting from park to drive was one of the first signs something was wrong with my transmission control unit. The good news is you caught this early, right after your service. With my Acadia, waiting too long led to more extensive transmission repair costs. The total came to 1500 Euro because I delayed addressing the initial symptoms. A few quick checks you can do before visiting a shop: Verify transmission fluid level and condition, Check if the neutral position sensor connector is properly secured, Look for any obvious wiring damage near the gear selector Based on the error codes and symptoms matching my experience, I strongly suspect your transmission control module needs attention. Get it checked soon to avoid more serious transmission issues developing. A certified transmission repair specialist should be able to confirm if the control unit needs replacement or if there are underlying mechanical problems with your automatic transmission.

Thanks for the detailed information about your Acadia case. After checking my Terrain based on your suggestions, I found some concerning signs. The transmission fluid looks darker than normal, and I noticed a loose connection near the gear selector that might explain the shift lock issues. The error codes P0700 and P0706 matching your experience definitely gives me a better idea of what to expect. Your warning about increased transmission repair costs from waiting makes sense, I will schedule an appointment with a transmission specialist this week. The fact that your transmission failure symptoms also got worse with temperature is particularly relevant. The gear linkage problems you mentioned with the control unit seem very similar to what I am experiencing with my automatic transmission. Do you recall if your repair shop had to do any additional programming after replacing the control unit? I want to be prepared for potential costs beyond just the parts. Looking at around 1500 Euro for repairs based on your case seems reasonable if it resolves all these transmission issues.