Run through these steps one by one. Do not rush to replacing ECM’s for this issue, wiring harnesses or any parts for that matter. There are several things that should be done in chronological order to diagnose this issue. The issue is most commonly initiated due to keyless entry/start interference and ends up with an incorrect throttle pedal that compounds the issue.
1. Inspect the throttle body for carbon build up. Clean and perform a Throttle Angle relearn (Bulletin D11-02 shown below). Even if they replace the throttle body they should still perform the relearn or the P2138 code will persist.
2. Verify the pin colors inside the APP (throttle pedal) match the pin colors on the APP connector. There are two different pedals used on 2010 and 2011 model chassis. Part number 20844306 has gold plated terminals while 25955673 has tin colored terminals. The terminals colors must match the connector. I’ve found several where the mechanic installed the wrong APP which resulting in more issues as mixing the pin color types (materials) will cause fretting corrosion resulting in resistance issues. That will also cause a derate and P2138 APP Correlation code. The correct part number should be looked up by VIN. Somehow the UPS system allows the field to order the wrong pedal.
3. (See attachment) Bypass keyless start by unplugging the keyless harness from the OEM ignition switch, unplug the OEM connector from the green connector then return the OEM connector to the OEM ignition switch. The keyless systems relays and module interrupts the ignition voltage feed to the ECM, which can cause an array of issues. Typically, on the W62 chassis an engine derate and P2138 code is the first to appear but we’ve also seen a few cases where the rear door lock sends a voltage spike forward through the BCM and ECM when it is activated resulting in more issues that can also cause the P2138 code. Operate car for a few days with keyless disconnected to validate. Bypassing keyless is intended to be a short term diagnostic aid, not left disconnected permanently. Typically, bypassing keyless fixes the P2138 code and derate issue. The unfortunate situation is by the time the mechanic does this he has already tossed on a few pedals (and a good chance he installed the wrong pedal while doing so resulting in more problems), throttle bodies, and ECM’s. Do not replace an ECM for a P2138 code.
4. Open and inspect the 40 way connector (square connector with a bolt in the center) located between the dash and body corner glass for debris, spilt coffee, aluminum shavings, etc. This is the main throwaway between the interior circuits and ECM. If there is a resistance issue in this plug there will be issues with the APP.
5. Disconnect the main connector plugged into the throttle body and inspect the wires near the connector for breaks. UPS bulletin D09-25. It is also advisable to use a pocket screw driver to open the connector to inspect the wires inside the terminals inside the connector. It is possible a wire could be hanging on to a terminal by a single strand of wire.
6. Last resort, remove the entire interior chassis harness as well as keyless start harness from the cab and wrap them with aluminum reflective tape or some type of EMI shielding material. The idea is the 5 volt reference signal is picking up interference from another high power source such as the blower motor circuit. 3M™ Embossed Tin-plated Copper Foil 1345, 3/8 in x 18 yd is great for this.