I have found that most of these type of complaints stem from a leak of some sort from sunroof...ect. Here is a common TSB that give guidance on the subject.
Let me know if it helps you.
TECHNICAL
Bulletin No.: 07-06-04-019A
Date: November 06, 2008
Subject:
Intermittent MIL/SES, DTC P2138 with Reduced Engine Power (Repair Instrument Panel (IP) to Body Harness Connector)
Models:
2005-2008 GM Passenger Cars and Light Duty Trucks (Including Saturn)
2005-2008 HUMMER H2
2006-2008 HUMMER H3
2005-2008 Saab 9-7X
Supercede:
This Bulletin is being revised to add additional diagnostic service information. Please discard Corporate Bulletin Number 07-06-04-019 (Section 06 - Engine/Propulsion System)
Condition
Some customers may comment on an intermittent MIL/SES light Illuminated with a message center or an indicator that displays Reduced Engine Power. DTC P2138 Accelerator Pedal Position (APP) Sensor 1-2 Correlation may be set. The voltage difference between APP sensor 1 and APP sensor 2 exceeds a predetermined value for more than a brief period. Refer to DTC P2138 in SI for further diagnostics.
Cause
On some vehicles, the IP to body harness connector, which carries the APP signal to the ECM/PCM, is located in the left hand kick panel area or inside the IP. Water intrusion into these areas could cause an electrical short in the APP circuits setting a P2138 code.
Correction
Note: Make sure that no add-on/aftermarket equipment is electrically connected to the APP signals or to the other ECM/PCM five volt reference or return lines. These can generate false P2138 and/or other ETC System DTCs. If there are other engine DTCs besides P2138, such as MAP sensor faults (i.e. P0107, P0108), resolve these first before attempting to diagnose P2138 DTCs.
Important: Refer to DTC P2138 in SI to confirm that the ETC pedal is operating correctly.
Locate the IP to body harness connector, which may be located in and around the left hand kick panel area or inside the IP. Check to see if there is a water leak into the area and repair as necessary. Use a water hose to determine the source of a water leak and effectiveness of repair. Some examples of potential water leaks are: A-pillar seals, sunroof drain lines (if so equipped) and windshield/cowl sealing. Finally, repair or replace the IP to body harness connector, making sure no corrosion or debris is left on or between the terminals.
Let me know if it helps you.
TECHNICAL
Bulletin No.: 07-06-04-019A
Date: November 06, 2008
Subject:
Intermittent MIL/SES, DTC P2138 with Reduced Engine Power (Repair Instrument Panel (IP) to Body Harness Connector)
Models:
2005-2008 GM Passenger Cars and Light Duty Trucks (Including Saturn)
2005-2008 HUMMER H2
2006-2008 HUMMER H3
2005-2008 Saab 9-7X
Supercede:
This Bulletin is being revised to add additional diagnostic service information. Please discard Corporate Bulletin Number 07-06-04-019 (Section 06 - Engine/Propulsion System)
Condition
Some customers may comment on an intermittent MIL/SES light Illuminated with a message center or an indicator that displays Reduced Engine Power. DTC P2138 Accelerator Pedal Position (APP) Sensor 1-2 Correlation may be set. The voltage difference between APP sensor 1 and APP sensor 2 exceeds a predetermined value for more than a brief period. Refer to DTC P2138 in SI for further diagnostics.
Cause
On some vehicles, the IP to body harness connector, which carries the APP signal to the ECM/PCM, is located in the left hand kick panel area or inside the IP. Water intrusion into these areas could cause an electrical short in the APP circuits setting a P2138 code.
Correction
Note: Make sure that no add-on/aftermarket equipment is electrically connected to the APP signals or to the other ECM/PCM five volt reference or return lines. These can generate false P2138 and/or other ETC System DTCs. If there are other engine DTCs besides P2138, such as MAP sensor faults (i.e. P0107, P0108), resolve these first before attempting to diagnose P2138 DTCs.
Important: Refer to DTC P2138 in SI to confirm that the ETC pedal is operating correctly.
Locate the IP to body harness connector, which may be located in and around the left hand kick panel area or inside the IP. Check to see if there is a water leak into the area and repair as necessary. Use a water hose to determine the source of a water leak and effectiveness of repair. Some examples of potential water leaks are: A-pillar seals, sunroof drain lines (if so equipped) and windshield/cowl sealing. Finally, repair or replace the IP to body harness connector, making sure no corrosion or debris is left on or between the terminals.
