2104 is a relatively rare OBD-II trouble code. It indicates that your GMC Terrain’s PCM has disabled the gas pedal, leaving the engine stuck at idle. It’s a generic code, so it has the same definition no matter what type of vehicle has the code.
P2104 Definition: Throttle Actuator Control System – Forced Idle
There are two parts to the P2104 definition:
1. Throttle Actuator Control System
Your GMC Terrain uses a “drive by wire” throttle system, which allows the PCM to control the throttle electronically. The advantage of this system is that the computer can control engine torque. This can increase engine longevity and decrease wear and tear on the drivetrain.
The system that controls the throttle motor is referred to as the Throttle Actuator Control (TAC) system. It takes input from the gas pedal and runs it through your Terrain’s PCM (powertrain control module) before the throttle input gets to the throttle blade.
2. Forced Idle
Forced idle means that the PCM is ignoring throttle input. It can do this for a few different reasons (covered below). It only happens when something is seriously wrong.
P2104 is closely related to P2106 (limited power instead of stuck idling).
P2104 Symptoms: GMC Terrain
Check Engine Light
While you clearly know that your Terrain’s check engine light is on (that’s why you’re here, after all), there should always be at least one more code (other than P2104).
P2104 is letting you know that the engine is idling on purpose. However, it does not give any indication as to what is wrong with the vehicle. The other code(s) will tell you what caused the engine to go into this state. You’ll want to use those codes to narrow down what is going on.
Engine Stuck at Idle
The throttle pedal will be completely inoperable with P2104.
Other
Since another trouble code always accompanies P2104, there may be other symptoms.
GMC Terrain P2104 Causes + Diagnosis
Remember, P2104 is not reporting a problem with your Terrain itself. Rather, it is telling you that it has responded to something that it considers to be an emergency. There will be another code (or codes) that will give you more to go on, and that’s where you should start your diagnosis.
Here’s what you can do with just P2104:
1. Look for TSB’s
Technical service bulletins are memorandums that GMC (and all manufacturers) share detailing common problems with a particular vehicle. They can help a lot.
Follow this link to the NHTSA site. Type in your model year, GMC, and Terrain in that order. You will see “manufacturers communications.” That’s what you are looking for.
Check to see if there are any bulletins related to the TAC system or throttle system in general. If there are, reference it with the other codes that have been thrown, and you’ll have a serious head start in your diagnosis.
3. Check for Other Codes
The only thing left to do is check for other codes. P2104 lets you know that your Terrain’s PCM has responded to an issue by entering fail-safe mode.
There will almost certainly be other codes that appear with this one. Check them. Once you fix the issue causing them, P2104 will take care of itself.
Conclusion
P2104 is letting you know that your Terrain has disabled the throttle on purpose. Use the other trouble codes that you pulled to fix the problem. Once you do that, throttle control will return.