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2007 Pontiac Grand Prix P2138 Code: Wet Weather Only Reduced Engine Power & Cruise Control Failure

Model: Grand Prix Fault Code: P2138 Posted: 2014-12-23 11:56

Hi everyone, I own a 2007 Pontiac Grand Prix with a sunroof that has been throwing DTC P2138 for nearly a year. The issue only occurs during rainy or wet road conditions, when the 'Reduced Engine Power' message appears on the dashboard along with the check engine light. It does not happen on dry days. I've already replaced the accelerator pedal assembly multiple times and removed the kick panel to inspect for visible leaks. I’ve also checked all wire harnesses from the pedal and found no signs of moisture or damage. Despite reviewing numerous forum posts, I haven’t been able to resolve this issue. Could this be related to water intrusion in the instrument panel (IP) to body harness connector? The P2138 code indicates a correlation failure between Accelerator Pedal Position (APP) Sensor 1 and APP Sensor 2. When moisture enters this connection, it can cause a voltage difference that exceeds the ECM’s threshold, triggering the DTC. I’ve read about GM Technical Bulletin 07-06-04-019D (June 28, 2010), which specifically addresses intermittent MIL illumination with P2138 in vehicles from 2005 to 2011. The bulletin recommends inspecting for water leaks around the A-pillar, sunroof drains, and windshield/cowl seals. It also advises checking for corrosion at the IP-to-body harness connector. I’ve tried using a scan tool to monitor APP sensor data — one sensor appears to be out of sync with the other, which confirms the 'correlation' issue described in the trouble tree. I've also noticed that cruise control fails simultaneously during wet weather, which may be linked to throttle system malfunction due to incorrect pedal position readings. I’m now considering sealing potential water entry points — including a harness junction box under the hood on the driver’s side (visible when opening the hood) — with dielectric grease and electrical tape. I’ve already done this successfully in my own vehicle, and the issue has not returned since. If anyone has experience with P2138 on a 2007 Grand Prix or similar GM models, especially with wet-weather-only symptoms, any insights would be greatly appreciated. I’d also like to know if there are known fixes involving harness sealing or sensor recalibration.

Related fault codes

P2138

Comments (9)

Anonymous 2014-12-23 13:52

Technical Bulletin No.: 07-06-04-019D (June 28, 2010) Subject: Intermittent MIL Illumination with DTC P2138 and Reduced Engine Power Models: 2005–2011 GM Passenger Cars & Light Trucks (including Saturn), 2005–2009 Saab 9-7X, 2006–2010 HUMMER H3/H2 This bulletin addresses water intrusion into the instrument panel (IP) to body harness connector, which carries signals from the Accelerator Pedal Position (APP) sensors to the ECM/PCM. When moisture enters this connection, it causes a voltage discrepancy between APP Sensor 1 and APP Sensor 2 — exceeding a calibrated threshold for more than a set period — triggering DTC P2138. Key Steps: 1. Verify no aftermarket devices are connected to APP sensor circuits or ECM/PCM 5V reference lines (refer to SI: Checking Aftermarket Accessories). 2. Run Diagnostic System Check – Vehicle. - If any 5V reference DTCs appear, consult the DTC List – Vehicle. - If not, proceed to Step 3. 3. Locate the IP-to-body harness connector — typically in or near the left kick panel or inside the instrument panel. Refer to Wiring Systems or Power and Signal Distribution in SI. 4. Inspect for water leaks: A-pillar seals, sunroof drains (if equipped), windshield/cowl gaskets. - Use a water hose to simulate wet conditions if needed. 5. Check terminals for corrosion or debris — repair as necessary. 6. After repairs, verify system function using Diagnostic Repair Verification or DTC List – Vehicle. Note: Aftermarket equipment can generate P2138. Warranty applies only when the root cause is a water leak (excluding Saab U.S. models).

Anonymous 2014-12-23 14:45

Thanks for sharing, autojoe — I’ve reviewed this bulletin and attempted several fixes already, but haven’t resolved the issue. I’m hoping someone with identical symptoms can share their actual fix or diagnostic steps.

Anonymous 2014-12-23 16:38

Vehicle is stock with no other DTCs reported. The P2138 code relates to Accelerator Pedal Position (APP) Sensor 1-2 Correlation. The ECM uses two sensors in one assembly, each receiving a separate 5V reference from the ECM. These references are internally connected but supplied via distinct terminals. APP Sensor 1 sends a signal indicating pedal position; the ECM uses this to control throttle actuation. When the ECM detects a mismatch between the two sensors — meaning they don’t agree on pedal position — it sets DTC P2138. This can result in engine derating and cruise control deactivation. A high-end scan tool with live data monitoring is required to verify if one sensor's voltage curve deviates from expected values.

Anonymous 2014-12-23 23:21

Thanks, kev2 — vehicle is stock and no other codes have been present. I didn’t mention earlier that cruise control also fails during wet weather. This may be directly related to the throttle system malfunction caused by incorrect pedal position readings. Next time the code appears, I’ll scan for all DTCs to ensure nothing else is involved.

Anonymous 2014-12-24 16:04

The PCM detects a TAC (Throttle Actuation Control) issue due to inconsistent APP sensor data — this often leads to cruise control disabling. The key term here is 'correlation' — the two sensors must agree on pedal position. If one sensor reads incorrectly, the ECM flags it as an error and disables power delivery. To diagnose: Use a high-end OBD2 scanner with live data to view APP Sensor 1 and APP Sensor 2 voltage readings. One should follow a smooth, predictable curve — if not, that indicates a faulty sensor or wiring issue.

Anonymous 2014-12-29 10:40

Sorry for the delay. I packed two harness sections (attached with plastic clips on driver-side strut) and applied dielectric grease. After waiting until heavy frost caused the system to trigger again, I scanned: P2138, P2125, and P1125. I’m now researching possible causes — but any input would be helpful. Thanks for the help!

Anonymous 2017-05-01 10:45

07grandprix wrote: Sorry for the delay. I packed two harnesses, both attached with plastic clips on driver side strut, with dielectric grease and waited for it to throw codes again. They threw this morning because of the heavy frost we had here. Got codes checked again: P2138; P2125; and P1125. I'll be doing research today but please let me know your thoughts too. Thanks Did you ever find a permanent fix? My 2006 Grand Prix is showing the same symptoms — any advice on how to proceed?

Anonymous 2017-05-01 16:05

Yes, I found it. There’s a harness junction box under the hood on the driver's side, located just above the engine. It’s visible when you open the hood. After applying dielectric grease and sealing it with electrical tape, the issue has not returned since. This appears to be a key water entry point that affects APP sensor signal integrity. Best of luck to others facing this problem!

Anonymous 2019-04-26 07:16

Thanks for sharing! I have a similar issue on my 2009 Pontiac G6 3.5L Flex. I’ve already checked the sunroof drain, replaced TPS and accelerator pedal sensor — but now I suspect a harness junction near the ABS manifold block. While it’s typically used by ABS, it may still be involved in throttle control due to shared wiring or signal interference. This could explain why cruise control disables during wet weather. I’ll inspect that area next.