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Tom, ASE Certified Technician
Category: GMC
Satisfied Customers: 25920
Experience:  ASE Certified Master Technician
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Reduced power and check engine light, Throwing both P2120 &

Customer Question

Reduced power and check engine light
JA: Does the light stay on solid or does it flicker? Any noticeable performance problems lately?
Customer: Throwing both P2120 & P1125 Codes No performance delays or flickering
JA: What is the model/year of your GMC?
Customer: 2005 2500
JA: Are you hoping to fix this yourself? What have you tried so far?
Customer: replaced the gas cap & vapor valve
JA: Anything else you think the mechanic should know?
Customer: Checked wire connections
JA: OK. Got it. I'm sending you to a secure page on JustAnswer so you can place the $5 fully-refundable deposit now. While you're filling out that form, I'll tell the GMC Mechanic about your situation and then connect you two.
Submitted: 3 months ago.
Category: GMC
Expert:  Tom replied 3 months ago.

Whats the code stored for the check engine light ?

Customer: replied 3 months ago.
What is this? I plugged the scanner in and these two codes came up
Expert:  Tom replied 3 months ago.

Sorry that didn't come up before let me check something for you I will post back in a few minutes.

Expert:  Tom replied 3 months ago.


The accelerator pedal assembly contains 2 individual APP sensors within the assembly. The APP sensors 1 and 2 are potentiometer type sensors, each with the following circuits:

  • A 5-volt reference circuit
  • A low reference circuit
  • A signal circuit

The APP sensors are used to determine the pedal angle. The control module provides each APP sensor a 5-volt reference circuit and a low reference circuit. The APP sensors then provide the control module with signal voltage proportional to pedal movement. Both APP sensor signal voltages are low at rest position and increase as the pedal is applied.

  • Remove the connector position assurance (CPA) retainer.
  • Disconnect the accelerator pedal position (APP) sensor electrical connector

Expert:  Tom replied 3 months ago.

This is the most common issue going to post moe info hang in there.

Expert:  Tom replied 3 months ago.

Did you test the APP sensor this is usually the issue for this problem.

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