Throttle/Pedal Position Sensor/Switch “D/E” Voltage Correlation

What P2138 means

Electronic throttle control systems (ETCS) replace the throttle cable with an accelerator pedal position (APP) sensor mounted on the pedal bracket. This sensor uses two independent Hall-effect circuits: VPA (the primary signal for engine control) and VPA2 (the secondary signal for self-monitoring). At rest, VPA reads approximately 0.5-1.1 V and VPA2 reads approximately 1.2-2.0 V; at full pedal depression, VPA reads 2.6-4.5 V and VPA2 reads 3.4-4.75 V. The ECM continuously compares both signals. P2138 is set as a 1-trip code when either condition holds for 2.0 seconds: (a) the difference between VPA and VPA2 drops to 0.02 V or less, suggesting the two signal circuits have shorted together; or (b) VPA drops to 0.4 V or less AND VPA2 drops to 1.2 V or less simultaneously, suggesting a shared reference supply or both grounds have failed. The ECM immediately enters fail-safe mode: if one circuit is bad it limits engine output; if both circuits fail it locks the throttle near idle and blocks further acceleration.

Symptoms

• Check engine light ON -- illuminates immediately as P2138 is a 1-trip code
• Sudden severe loss of power -- engine enters full fail-safe with throttle locked near idle (no acceleration possible beyond low speed)
• Accelerator pedal unresponsive or only partially responsive regardless of pedal pressure
• Engine idles normally but will not rev above a low ceiling when the pedal is depressed
• Possible stall if attempting to maintain highway speed or merge with traffic in fail-safe mode

Common causes

• Short circuit between the VPA and VPA2 signal wires causing both signals to converge to the same voltage -- most common electrical cause
• Failed accelerator pedal assembly -- internal short or open in the dual Hall-effect sensor element inside the pedal bracket
• Open circuit in the VCPA (5 V reference) wire, causing both VPA and VPA2 to drop to near 0 V simultaneously
• Open in the EPA or EPA2 ground wires, causing both signals to float and converge
• Corroded or spread pin contacts at the APP sensor 6-pin connector
• Water intrusion in the pedal harness connector (floor mat water pooling around the pedal area)

Severity & driving advice

Severity: High — Full fail-safe locks the throttle near idle -- the vehicle cannot accelerate safely at highway speed. Do not drive on motorways. Repair immediately.

Can I drive? Do not drive on highways. Limp-home only. Repair immediately.

Diagnostic approach

1. Read live APP sensor signals on a scan tool to identify the fault pattern — Connect the scan tool and navigate to Accelerator Sensor Out No. 1 (VPA) and No. 2 (VPA2). With the pedal released, VPA should read 0.5-1.1 V and VPA2 should read 1.2-2.0 V, with a difference greater than 0.02 V. At full pedal depression, VPA should read 2.6-4.5 V and VPA2 should read 3.4-4.75 V. If both read near-zero at all positions, the shared reference supply (VCPA) is open. If both readings match closely at the same voltage, the VPA and VPA2 signal wires are shorted together.
2. Inspect the APP sensor connector for corrosion or damage — Disconnect the 6-pin APP sensor connector at the pedal bracket (accessible under the dash at the driver footwell). Inspect each terminal for green corrosion, spread pins, bent tabs, or water contamination. The connector is a common moisture-entry point on vehicles in wet climates or where floor mats allow water to pool near the pedal. Clean terminals with electrical contact cleaner.
3. Measure harness continuity and shorts with both connectors unplugged — Disconnect both the APP sensor connector and the ECM connector. Measure resistance from VPA at the APP harness to VPA at the ECM -- below 1 ohm expected. Measure from VPA2 at the APP harness to VPA2 at the ECM -- below 1 ohm. Then measure directly from VPA to VPA2 in the harness (both connectors unplugged) -- should be above 10 kohm. A near-zero reading between VPA and VPA2 confirms a wire-to-wire short in the harness between the pedal and the ECM.
4. Check ECM reference voltages at the APP sensor connector — With the APP sensor connector unplugged and ignition ON, measure voltage from the VCPA pin to the EPA (ground) pin at the harness connector -- should read 4.5-5.5 V. Measure from VCP2 to EPA2 -- same spec. Zero volts on either reference confirms an open in the ECM reference driver or a short on the reference wire. If reference voltages are correct but signals still correlate incorrectly, the internal sensor element has failed.
5. Replace the accelerator pedal assembly if circuit checks pass — If harness continuity, inter-circuit isolation, and reference voltages are all within spec but signals still correlate incorrectly, the internal Hall-effect sensor has failed. On most vehicles the APP sensor is not serviceable separately -- the entire accelerator pedal assembly is replaced. After replacement, clear codes and perform a confirmation drive cycle including full pedal sweep to verify VPA and VPA2 readings are in spec and the monitor passes. On Toyota vehicles also perform the pedal calibration: ignition on without pressing the pedal, wait 2 seconds, ignition off for 10 seconds.

Make & model notes

Toyota: Toyota ETCS systems on FJ Cruiser, 4Runner, Tacoma, Camry, and most post-2003 models use a 6-wire APP sensor (VCPA, EPA, VPA, VCP2, EPA2, VPA2). After pedal assembly replacement, Toyota requires an accelerator pedal calibration: turn ignition on without pressing the pedal, wait at least 2 seconds, turn ignition off and wait 10 seconds, then start the engine. This resets the ECM's learned pedal-zero position before the confirmation drive cycle.

Nissan/Infiniti: Nissan Altima, Maxima, Pathfinder, and Infiniti QX models with ETCS frequently develop P2138 from corrosion in the APP sensor connector under the driver's dash. The 6-pin connector on earlier examples lacks a rubber seal and is exposed to road spray entering through the firewall pass-through. Cleaning or replacing the connector often resolves the code without replacing the pedal assembly.

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