P2135 Code: Throttle Position Sensor 'A'/'B' Voltage Correlation

Quick Answer

• What it means: The engine computer has detected a mismatch in the voltage signals between Throttle Position Sensor A and Throttle Position Sensor B.
• Severity: High. Your vehicle will likely enter a heavily restricted "limp mode." You should pull over safely and get the vehicle towed, as it may lose acceleration completely.
• Most common cause: A failing or defective Throttle Position Sensor (TPS) or a completely failed electronic throttle body assembly.

What the P2135 Code Means

In modern vehicles equipped with Electronic Throttle Control (ETC)—often called "drive-by-wire"—there is no physical cable connecting your gas pedal to the engine's throttle body. Instead, when you press the accelerator, sensors tell the Powertrain Control Module (PCM) how far you've pressed the pedal, and the PCM commands an electric motor on the throttle body to open the throttle plate accordingly.

Because a malfunctioning electronic throttle could cause a runaway vehicle, manufacturers build in redundancy. The throttle body uses two independent Throttle Position Sensors (Sensor A and Sensor B) to monitor the exact angle of the throttle plate. The PCM constantly compares the voltage signals from these two sensors. Depending on the manufacturer, these signals might move in opposite directions (e.g., as Sensor A goes from 0.5V to 4.5V, Sensor B goes from 4.5V to 0.5V) or move together at a specific ratio.

If the PCM detects that the voltages from Sensor A and Sensor B do not correlate or match the expected mathematical relationship for a set amount of time (usually less than a second), it triggers code P2135: Throttle Position Sensor 'A'/'B' Voltage Correlation. To protect the driver, the PCM immediately restricts engine power.

Common Symptoms

When a P2135 is triggered, the PCM takes immediate defensive action. You will almost certainly notice:

• Illuminated Check Engine Light: Often accompanied by a wrench icon or a "Reduced Engine Power" warning message on the dashboard.
• Limp Mode: The vehicle will severely limit acceleration and top speed (often capping speed at 10-20 mph).
• Lack of Acceleration: Pressing the gas pedal may do absolutely nothing.
• Engine Stalling or Rough Idle: The engine may struggle to maintain a steady RPM or shut off entirely.
• High Idle: In some cases, the PCM may command a slightly higher than normal idle to keep the engine running while disabling the gas pedal.

Common Causes

Ranked from most to least likely, the causes of a P2135 code include:

1. Defective Throttle Body Assembly: On most modern cars, the TPS is permanently integrated into the electronic throttle body. If the internal sensors wear out or fail, the entire unit must be replaced.
2. Damaged Wiring Harness: Chafed wires, broken insulation, or melted wiring near the throttle body can cause short circuits or open circuits, skewing the sensor voltages.
3. Corroded or Loose Connectors: Moisture, dirt, or a loose pin in the electrical connector at the throttle body can create high resistance and corrupt the sensor signals.
4. Carbon Buildup in the Throttle Body: Severe carbon buildup can restrict the throttle plate's movement, causing the motor to draw excess current and skewing the sensor readings (though this usually triggers mechanical binding codes first).
5. Faulty PCM: Very rarely, the engine computer itself may have an internal fault that prevents it from reading the sensor voltages correctly.

How a Technician Diagnoses It

An ASE-certified technician will take a methodical approach to diagnose a P2135 code, ensuring they don't replace an expensive throttle body if a simple wire is broken.

1. Scan and Check TSBs: First, they will pull all codes and freeze-frame data. They will also check for Technical Service Bulletins (TSBs). (Note: GM vehicles, for example, have numerous TSBs related to P2135 requiring specific wiring harness repairs or throttle body replacements).
2. Live Data Analysis: Using an advanced scan tool, the technician will graph the live data for TPS A and TPS B voltages. By slowly pressing the accelerator pedal with the engine off (Key On, Engine Off), they will watch for glitches, dropouts, or spikes in the voltage graph. A smooth, linear change is good; a sudden jagged drop to 0V is bad.
3. Visual Inspection: The technician will closely inspect the wiring pigtail and connector at the throttle body. They will look for bent pins, corrosion (green crust), or wires that have been chewed by rodents or chafed against the engine block.
4. Multimeter Testing: If the scan tool shows erratic voltage, the technician will use a digital multimeter to probe the 5-volt reference wire, the ground circuit, and the signal wires at the throttle body connector to verify the PCM is sending the correct voltage and that the ground is solid.
5. Throttle Body Inspection: They will remove the intake boot to inspect the throttle plate for excessive carbon buildup or signs of internal gear failure.

Typical Repair Cost

Because the Throttle Position Sensor is usually built directly into the electronic throttle body on modern vehicles, the most common fix is replacing the entire throttle body assembly.

• Throttle Body Replacement: $200 to $600 (Parts and labor)
• Wiring Repair / Connector Pigtail Replacement: $100 to $250
• Throttle Body Cleaning and Re-learn: $100 to $150 (If the issue was purely carbon binding, though less common for a correlation code).

Prices vary based on the make and model of the vehicle and local labor rates.

Can I Keep Driving with this Code?

No. It is not safe to keep driving with a P2135 code. When this code is active, your vehicle is experiencing a critical failure in the electronic throttle system. The PCM will force the car into limp mode, meaning you will not have enough power to merge onto a highway, climb a hill, or maneuver safely in traffic. Furthermore, the throttle could become completely unresponsive at any moment, leaving you stranded. If this code appears while driving, pull over to a safe location immediately and arrange for a tow.

Related Codes

• P0120: Throttle/Pedal Position Sensor 'A' Circuit
• P0121: Throttle/Pedal Position Sensor 'A' Circuit Range/Performance
• P0122: Throttle/Pedal Position Sensor 'A' Circuit Low
• P0220: Throttle/Pedal Position Sensor/Switch 'B' Circuit
• P0222: Throttle/Pedal Position Sensor/Switch 'B' Circuit Low
• P2138: Throttle/Pedal Position Sensor/Switch 'D'/'E' Voltage Correlation (relates to the gas pedal sensors rather than the throttle body)

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