P2135 Code: Throttle Sensor Voltage Mismatch — Causes & Fix

P2135 means the two throttle position sensors inside your electronic throttle body disagree with each other by more than the engine computer allows. Because it can no longer be sure how far the throttle is open, the computer usually limits engine power, which is why P2135 so often comes with a “reduced engine power” message, an EPC or wrench light, or a car that will barely accelerate. The fault is real, but it is not always the throttle body. A corroded connector, a damaged wire or a shared reference circuit can cause it, and manufacturers’ own bulletins disagree about whether a new throttle body is the right fix. This article covers what sets P2135, the six real causes in order of likelihood, the live-data test that narrows it down, and what each repair typically costs.

Start with the table, then read the triage note under it.

Cause	Telltale Sign	Typical Cost
Connector or wiring fault at the throttle body	Code comes and goes; one sensor signal drops out when the harness is moved	$20 to $300
Worn or failed throttle position sensor	One sensor’s live reading glitches or flat-spots at a particular throttle angle	$250 to $800 (usually a throttle body)
Known throttle body defect covered by a bulletin	Your vehicle is listed in a manufacturer bulletin for P2135	$0 to $800
Shared 5-volt reference or ground fault	Other sensor codes at the same time, such as pedal or pressure sensor codes	$50 to $300
Damage from cleaning or water	Code started after the throttle body was cleaned or the engine bay was washed	$0 to $800
Engine computer fault	Sensors, wiring and reference all test good	$300 to $1,000

Stop driving, or drive only to get off the road, if the car is in reduced-power mode. With engine power limited, the car may not be able to accelerate enough to merge, overtake or climb safely, and on some vehicles the engine can stall. If P2135 is stored but the car drives normally, it can be driven carefully to a diagnosis, but expect it to drop into reduced power without warning.

What P2135 Actually Means

With electronic throttle control, sometimes called drive-by-wire, there is no cable between the accelerator pedal and the throttle. The engine computer reads the pedal and opens the throttle blade with an electric motor. Because a throttle that opens unexpectedly is dangerous, the throttle’s position is measured by two sensors, not one, and the computer constantly checks them against each other. The two are designed to read differently. On many throttle bodies one signal rises as the throttle opens while the other falls, or both rise at different rates, so the same fault is unlikely to affect both in the same way.

P2135 is “throttle/pedal position sensor/switch A/B voltage correlation”. On most vehicles, A and B are the two throttle body sensors. The accelerator pedal has its own pair and its own correlation code, P2138. General Motors’ service bulletin 12-06-04-003A states the logic plainly: P2135 “may be caused by the difference between the TP Sensor 1 and TP Sensor 2 exceeding a calibrated value for more than 2 seconds.” When that happens, the computer cannot tell which sensor is right, so it protects the engine and the driver by limiting how far it will open the throttle.

Do Not Assume It Is the Throttle Body

This is the point that saves money. On most modern engines both sensors are built into the throttle body, so the standard parts-counter answer to P2135 is a new throttle body, often several hundred dollars. Sometimes that is right. But the manufacturers’ own bulletins show it is not a safe assumption.

General Motors issued two bulletins on P2135 with a reduced-power message. One, 12-06-04-003A, covers 2006 to 2011 cars and crossovers including the Chevrolet Malibu, Impala and Equinox, Pontiac G6 and Saturn Aura, with specific engines listed in the bulletin. The other, 11-06-04-007C, covers 2008 to 2011 trucks and SUVs including the Silverado, Sierra, Tahoe and Yukon. Both open with the same instruction, in capitals: “DO NOT replace the throttle body.” Instead, technicians clean the throttle body and replace the TP sensor cover and its terminals. On the trucks they also reprogram the engine computer afterwards.

Mazda reached the opposite conclusion for a specific build range of the 2009 to 2010 Tribute. Its service bulletin 01-007/13 attributes P2135, sometimes with P2111 or P0122 and symptoms such as rough idle and reduced power, to the throttle body assembly hardware, and the repair is a replacement throttle body. The lesson is not that one answer is right. It is that the correct repair depends on the vehicle, so check for bulletins and test the wiring before paying for a throttle body.

The Live-Data Test

A scan tool that shows both throttle position readings, usually labelled TP sensor 1 and 2 or TPS A and B, narrows the fault quickly. With the engine off and the ignition on, many drive-by-wire systems open the throttle as the pedal is pressed. Press the pedal slowly from closed to wide open and back, watching both readings or, better, graphing them. They should move smoothly and in their fixed relationship the whole way. Look for:

• A dropout or spike in one signal at a particular point: a worn spot on that sensor’s track, pointing at the sensor.
• A signal that jumps when the connector or harness is wiggled: a connector or wiring fault, not the sensor.
• Both signals shifted, with other sensor codes stored: a shared reference or ground problem.

Do not force the throttle blade open by hand with the ignition on, and keep fingers out of the throttle bore. The motor can move the blade with real force.

Symptoms You May Notice Alongside the Code

The main one is reduced power. Depending on the make, it shows as a “reduced engine power” message, an EPC light, a wrench light or simply a car that will not rev past a low limit. Others include a rough or low idle, hesitation or no response to the pedal, and occasional stalling. The problem is often intermittent at first, dropping into reduced power and recovering after the engine is restarted. The site’s article on the symptoms of a bad throttle position sensor covers these in more detail.

1. Connector or Wiring Fault at the Throttle Body

The throttle body sits on top of the engine, where its connector goes through constant heat cycling and vibration. A terminal that has lost its grip, green corrosion from moisture, or a harness rubbing against the intake can make one sensor signal flicker while the other stays steady, which is exactly what P2135 detects. Unplug the connector, inspect both halves for corrosion and spread or pushed-back terminals, and do the wiggle test in live data. With a wiring diagram, check the 5-volt reference, the ground and each signal wire for continuity back to the computer.

2. Worn or Failed Throttle Position Sensor

Older contact-type sensors use a wiper on a resistive track, and the track wears where the throttle spends most of its time. That produces a dead spot in one signal at a particular throttle angle. Newer non-contact sensors, typically Hall-effect, do not wear this way but can still fail electronically. Either way, the live-data sweep shows it as a glitch in one trace. On most modern throttle bodies the sensors are not sold separately, so the repair is a throttle body. The site’s guide to P0121 covers the related range fault for sensor A.

3. Known Throttle Body Defect Covered by a Bulletin

As the GM and Mazda bulletins show, some vehicles have a documented P2135 fault with a documented repair, which may or may not involve a new throttle body. Before buying parts, ask a dealer to check for service bulletins and software updates for your vehicle and engine. On some vehicles the throttle body and engine computer are covered by the emissions warranty for longer than the basic warranty.

4. Shared 5-Volt Reference or Ground Fault

Many engine computers supply the same 5-volt reference, and sometimes the same sensor ground, to several sensors. That can include the throttle position sensors, the accelerator pedal sensors and pressure sensors. If another sensor on that circuit shorts, or the shared ground develops resistance, the reference voltage sags and the throttle sensors’ signals shift. Because the two are designed to read differently, the shift can push them out of agreement. The signature is P2135 arriving with other sensor codes at the same time. Check the reference voltage at the throttle body connector, then unplug sensors that share it one at a time to find which is dragging it down.

5. Damage From Cleaning or Water

Throttle body cleaning is a common first step, and done carefully it is fine. But electronic throttle bodies contain the motor, gears and sensors, and solvent sprayed into the wrong place, or a throttle body soaked in cleaner, can damage them. The GM bulletins explicitly warn not to immerse electronic throttle control components in cleaning solvent of any type, not to drop them, and to replace the throttle body if it or its gear is dropped. A pressure-washed engine bay can also drive water into the connector. If P2135 appeared right after cleaning or washing, that is the likely cause. After any throttle body cleaning or replacement, most vehicles need an idle or throttle relearn procedure.

6. Engine Computer Fault

Rare, and the last conclusion. If both sensor signals look good at the throttle body connector with a meter or scope, the wiring and reference test good, and the computer still reports a disagreement, the fault may be in the computer’s inputs. Check for a software update first, because correlation limits are software values, and GM’s truck bulletin includes reprogramming as part of its fix.

Diagnostic Order

1. Record every stored code. Other sensor codes at the same time point to a shared reference or ground.
2. Check for service bulletins for your vehicle and engine before buying parts.
3. Inspect the throttle body connector and harness for corrosion, loose terminals and chafing.
4. Do the live-data sweep of both throttle position signals, with a wiggle test on the harness.
5. Check the 5-volt reference, ground and signal wires with a meter and wiring diagram.
6. Replace the throttle body only once the tests point at the sensor, and perform the relearn procedure afterwards.

Repair Cost

Typical costs, labelled as typical because throttle body prices vary widely between engines:

• Connector cleaning or terminal repair: roughly $20 to $150.
• Wiring or reference circuit repair: roughly $50 to $300.
• Electronic throttle body: roughly $250 to $800 installed, including the relearn. Some V8 and European throttle bodies cost more.
• Software update: often free under a bulletin or warranty, otherwise up to about $200.
• Engine computer: rarely needed; roughly $300 to $1,000 including programming.

Can You Keep Driving With a P2135 Code?

Not for long. P2135 affects the part of the engine management system that controls power, and the computer’s protective response, reduced power, can arrive at the worst moment, such as mid-merge. If the car is already in reduced power, drive only as far as needed to get somewhere safe. If the code is stored but the car is driving normally, get it diagnosed promptly and avoid situations that need full power.

Related Codes

• P2138: the same correlation check on the accelerator pedal’s two sensors. P2138 points at the pedal assembly or its wiring rather than the throttle body. See the P2138 guide and the symptoms of a bad accelerator pedal position sensor.
• P0121: throttle position sensor A range or performance. It is a plausibility problem with one sensor rather than a disagreement between two. See the P0121 guide.
• P0122, P0123, P0222 and P0223: throttle position sensor A or B circuit low or high. These are electrical faults on one sensor’s circuit and often appear with P2135.
• P2111 and P2112: throttle actuator control system stuck open or closed. They concern the motor and blade rather than the sensors, though Mazda’s bulletin shows P2111 can appear alongside P2135.
• P0641 and similar sensor reference codes: a fault on the 5-volt reference circuit. With P2135, they point to the shared reference.

Frequently Asked Questions

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