There is a GM Technical Service Bulletin (TSB) on this issue, below is a summary;
#07-06-04-019A: 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
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.
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