Throttle/Pedal Position Sensor/Switch “A/B” Voltage Correlation

What P2135 means

An electronic throttle body carries two independent position sensors in a single housing. On modern designs they are non-contact Hall-effect elements, so there is no wiper to wear out. Both output a rising 0-5 V signal as the plate opens, but they are deliberately offset so the ECM can cross-check one against the other. Signal A reports the actual angle; signal B rides higher and exists to catch a fault in A. At any given opening the factory characteristic is roughly signal-B x 0.8 approximately equal to signal-A + 1.11 V, so B sits about 1.4 V above A near idle (about 0.5-1.1 V on A versus 2.1-3.1 V on B released, and about 3.2-4.8 V versus 4.6-4.98 V at wide-open throttle). The two signals must never converge within 0.02 V of each other. P2135 sets when that relationship breaks: either the two signals collapse to within 0.02 V of one another for roughly half a second, or both signals drop into the low zone together (A at 0.2 V or less and B at 1.75 V or less) for about four-tenths of a second. The warning lamp comes on immediately, and the ECM cuts current to the throttle actuator so the return spring parks the plate near a 7-degree opening, then meters power through intermittent fuel-cut and ignition timing.

Symptoms

• Reduced engine power or limp-home mode, with the throttle plate held near its spring-return position so the vehicle will only creep
• Check-engine light plus, on many vehicles, a dedicated throttle or electronic-throttle warning lamp illuminating immediately
• Surging, hesitation, or a flat spot when you tip into the accelerator, because the ECM distrusts the pedal request
• Engine cutting to a forced idle, dying at stops, or refusing to rev past a low ceiling
• Unresponsive or laggy throttle pedal, sometimes with the idle stuck high or low after a restart

Common causes

• A failed throttle position sensor inside the throttle body, where one Hall-effect signal drifts, flatlines, or tracks the other too closely (most common, and the sensor is usually integral to the assembly)
• Corroded, spread, or loose terminals at the throttle-body connector, adding resistance that skews one signal relative to the other
• Chafed, pinched, or broken wiring between the throttle body and the ECM on the two signal circuits
• A degraded 5-volt reference feed or a poor sensor ground shared by both signals, which can pull both readings out of correlation at once
• A weak or faulty ECM throttle-sensor input or driver, considered only after the sensor, connector, wiring, reference, and ground all prove good

Severity & driving advice

Severity: High — Drive-by-wire fault that usually forces reduced-power limp mode; the engine can cut to idle or stall, so it is a real safety concern in traffic.

Can I drive? Limp mode likely - repair before normal driving.

Diagnostic approach

1. Confirm the code and read freeze-frame — Scan for all stored codes and capture the freeze-frame data from when P2135 set. Note any companion throttle codes such as P0120 through P0223, since a hard range or circuit fault on one signal generally takes priority and should be chased first. Freeze-frame also tells you whether the fault appeared at idle, on tip-in, or at wide-open throttle.
2. Watch both TPS signals live — With a scan tool, graph Throttle Position No. 1 and No. 2 while slowly sweeping the pedal from closed to wide open. Released, expect roughly 0.5-1.1 V on signal A and 2.1-3.1 V on signal B; at wide-open throttle expect about 3.2-4.8 V and 4.6-4.98 V. Both traces should climb smoothly with a fixed offset and never converge within 0.02 V. A momentary dropout, a flat segment, or the two lines touching pinpoints the failing signal.
3. Inspect the connector and harness — Unplug the throttle-body connector and check for corroded, bent, or backed-out terminals and any moisture intrusion. Measure each signal, the reference, and the ground circuit end-to-end from the connector to the ECM; each should read below 1 ohm. Then check each circuit to body ground, which should read 10 k-ohms or higher, confirming no short between wires or to ground.
4. Verify the reference and ground — With the key on, back-probe the sensor 5-volt reference feed, which should sit near 4.5-5.0 V, and confirm the sensor ground is clean and near 0 V. A sagging reference or a high-resistance ground pulls both signals off together and is a common reason both readings fall into the low zone at once, mimicking a bad sensor.
5. Replace, relearn, and retest — If the sensor is at fault it is normally replaced as part of the throttle-body assembly, since the elements are not separately serviceable. After any repair, perform the throttle and pedal relearn the vehicle requires, clear the code, then drive so the correlation monitor can run and confirm the signals hold their proper offset before returning the vehicle.

Make & model notes

Toyota: On Toyota electronic-throttle engines the two signals are VTA1 and VTA2, with the factory characteristic VTA2 x 0.8 approximately equal to VTA1 + 1.11 V. P2135 sets if they come within 0.02 V of each other for about 0.5 s, or both fall low (VTA1 0.2 V or less and VTA2 1.75 V or less) for about 0.4 s. Fail-safe parks the plate near 7 degrees. The sensor is integral to the throttle-with-motor body, so replace the assembly and run the throttle/pedal relearn with a Techstream.

Ford: On Ford drive-by-wire engines P2135 flags a TP sensor A/B correlation fault at the throttle body, where the two signals should track with a fixed offset. Check the 5-volt reference shared with other sensors first, because a dragged-down VREF can trip P2135 alongside other sensor codes. After throttle-body service perform the PCM idle/throttle relearn so the learned closed-throttle values reset.

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