The Nissan P2138 diagnostic trouble code (DTC) indicates a problem with the Throttle/Pedal Position Sensor/Switch “B” circuit high input. This article delves into diagnosing and resolving this code, particularly in the context of a 2013 Nissan Frontier with a 5.6L VK56DE engine swap from a Nissan Titan.
This specific case involves a custom intake manifold, a Nick Williams 102mm DBW throttle body (GM), 525CC injectors, and a Walbro lph255 fuel pump. While the swap was performed to maintain native compatibility, using a 2013 Titan ECM and a repinned harness, intermittent P2127 (Throttle/Pedal Position Sensor/Switch “A” Circuit Low Input) and P2138 codes persist. The issue manifests as a sudden loss of APP2 signal, requiring an engine restart or ECM reset. The truck operates normally until the code appears, with occurrences ranging from idling in the driveway to 300+ miles of driving.
Following the Nissan Factory Service Manual (FSM), all voltage validations and operational voltage changes check out. APP1, APP2, TPS1, and TPS2 connections at the ECM show correct voltages. The 6-pin Nissan connector to 6-pin GM connector pinout for the throttle body is confirmed correct, mirroring successful implementations in other Nissan trucks.
Meticulous inspection of the APP1, APP2, TPS1, and TPS2 wiring reveals no issues. The 5V reference and pedal assembly/throttle body voltages are within spec. Manipulating the wiring harness failed to induce the fault. All connections are clean, secure, and free of visible damage.
A new Odyssey Extreme AGM battery, maintained with a CTEK 4.3MUS charger, ensures proper voltage supply. All ground connections are clean and exhibit low resistance (.03V). Additional 4GA grounding wires supplement the factory grounding.
The following components have been replaced:
- Pedal assembly (containing APP1 and APP2)
- Throttle body (using both a 90mm ACDelco and two 102mm Nick Williams throttle bodies)
- ECM (using another 2013 Titan ECM)
Relays and fuses are confirmed functional. The EVAP system was investigated due to its shared 5V circuit with the APP/TPS sensors. Replacing the EVAP canister pressure sensor, despite previous related codes, yielded no improvement.
In one instance, after idling for 30 minutes at 80°F ambient temperature and 184°F coolant temperature, a brief idle dip preceded the P2138/P2127 codes. This raises the question: is this indicative of limp mode activation or a system event like EVAP purge?
This issue remains unique among similar 5.6L swapped Frontiers and Titan platforms using the same intake manifold and GM throttle body. The Frontier and Titan have near-identical APP and TPS reference ranges, ruling out incompatibility.
In conclusion, despite extensive troubleshooting, the intermittent P2138/P2127 codes persist. Potential remaining areas to investigate include:
- Thoroughly inspecting the wiring harness for intermittent shorts or opens, especially in areas subject to movement or vibration.
- Examining the engine harness connectors for potential corrosion or damage.
- Considering potential interference from aftermarket components or modifications.
- Consulting with a specialist familiar with Nissan engine swaps and custom tuning.
Resolving this issue requires a systematic approach to eliminate all possible causes. This case highlights the complexities of diagnosing intermittent electrical faults, even in meticulously executed engine swaps.
