P1255

What Does Code P1255 Mean?

The diagnostic trouble code P1255 indicates a detected fault within the Accelerator Pedal Position (APP) sensor circuit, specifically identifying that both the Pedal Demand Sensor 2 (PDS2) and Low Pedal Demand Sensor (LPDS) signals are registering below their calibrated minimum voltage thresholds. In modern drive-by-wire vehicles, the APP sensor assembly typically incorporates multiple redundant sensor elements (often two or three potentiometers or Hall effect sensors) to ensure safety and accuracy. These redundant signals, often designated as PDS1, PDS2, or LPDS/HPDS (Low/High Pedal Demand Sensor), provide the Powertrain Control Module (PCM) with precise information about the driver’s throttle input.

The PCM constantly monitors the voltage output from these individual sensor circuits. PDS2 and LPDS are designed to output a varying voltage signal proportional to the pedal’s physical position, typically ranging from a low voltage (e.g., 0.5V) at idle to a high voltage (e.g., 4.5V) at wide-open throttle. When the PCM detects that both the PDS2 and LPDS signals are consistently below a pre-defined low voltage threshold, indicating an unexpected lack of signal or a signal stuck at an abnormally low level, it interprets this as a critical failure in the pedal input. This condition affects the Electronic Throttle Control (ETC) system, as the PCM cannot accurately determine the driver’s requested engine power, leading to safety protocols being initiated.

Common Symptoms

• Malfunction Indicator Lamp (MIL) Illumination: The check engine light will be illuminated on the dashboard.
• Reduced Engine Power (Limp Mode): The vehicle’s engine power will be significantly restricted to prevent unintended acceleration or other unsafe conditions.
• Unresponsive or Inconsistent Throttle Pedal: The engine may not respond to accelerator pedal inputs, or the response may be delayed or erratic.
• Engine Cranks But Does Not Start: In some severe cases, if the PCM cannot establish a reliable pedal position, it may prevent the engine from starting.
• Rough or Erratic Idle: The engine may idle poorly or fluctuate irregularly as the PCM struggles to manage air-fuel mixture without accurate throttle input.
• No Cruise Control Operation: The cruise control system will be disabled due to the unreliable pedal input.

What Causes the Code P1255?

• Faulty Accelerator Pedal Position (APP) Sensor Assembly: Internal failure of the potentiometers or Hall effect sensors within the APP assembly, specifically affecting the PDS2 and LPDS circuits.
• Open or Shorted Wiring in the APP Sensor Circuit: Damage to the wiring harness leading to the APP sensor, causing an open circuit or a short to ground in the PDS2 and/or LPDS signal wires. This can also include issues with the 5-volt reference (VREF) or ground circuits shared by these sensors.
• Corrosion or Loose Connections: Contamination or poor seating of the electrical connector at the APP sensor or at the PCM, leading to high resistance or intermittent signal loss.
• Faulty Powertrain Control Module (PCM): While less common, an internal fault within the PCM affecting its ability to process APP sensor signals, supply VREF, or provide proper grounding can trigger this code.
• Physical Damage to the Accelerator Pedal Assembly: Although rare for this specific code, severe physical damage preventing proper mechanical operation of the pedal can indirectly affect sensor output.

How to Diagnose and Troubleshoot

Diagnosing P1255 requires a systematic approach using a suitable OBD-II scanner and a digital multimeter (DMM).

1. Retrieve and Document Freeze Frame Data: Connect an OBD-II scanner and retrieve code P1255. Note any associated freeze frame data, which provides vehicle operating conditions at the time the code was set. This can offer valuable clues regarding when the fault occurred. Clear the code after documenting.
2. Visual Inspection:
   • Inspect the accelerator pedal assembly for any physical damage, binding, or obstructions that might prevent full range of motion.
   • Locate the APP sensor connector. Visually inspect the wiring harness for any signs of chafing, cuts, pinches, or heat damage from the APP sensor back to the PCM. Pay close attention to areas where the harness passes through firewalls or near moving components.
   • Inspect the APP sensor and PCM connectors for bent pins, corrosion, or loose terminals.
3. OBD-II Scanner Live Data Analysis:
   • With the ignition in the “Key On, Engine Off” (KOEO) position, monitor the live data PIDS (Parameter IDs) for APP sensor PDS2 and LPDS.
   • Slowly depress the accelerator pedal through its full range of motion. Observe the voltage readings. For P1255, you would expect to see both PDS2 and LPDS signals stuck at or near 0 volts (e.g., <0.3V) regardless of pedal position.
   • Also, monitor other APP sensor signals (e.g., PDS1 if present) to see if they behave normally. Normal APP sensor signals should show a smooth, linear increase in voltage as the pedal is depressed.
4. Digital Multimeter (DMM) Testing at APP Sensor Connector:
   • Voltage Reference (VREF) Test: Disconnect the APP sensor connector. With KOEO, use a DMM to backprobe the VREF (usually 5V) pin at the harness side of the connector. Verify that a stable 5-volt reference signal is present. If not, troubleshoot the VREF circuit back to the PCM.
   • Ground Circuit Test: Using the DMM, check for good ground continuity at the ground pin of the APP sensor connector. Measure resistance between the ground pin and the vehicle’s chassis ground. It should be less than 5 ohms.
   • Signal Wire Voltage Test (Backprobe): Reconnect the APP sensor. With KOEO, carefully backprobe the PDS2 and LPDS signal wires at the connector. Slowly depress the pedal and observe the voltage readings. If the sensor is functioning correctly, you should see a smooth, increasing voltage sweep from low (e.g., 0.5V) to high (e.g., 4.5V). If both PDS2 and LPDS signals remain consistently low (e.g., near 0V), the sensor itself is likely faulty, or there’s an open circuit affecting the signal return to the PCM.
   • Continuity Test (Sensor to PCM): If VREF and ground are good at the sensor, and the sensor’s output appears correct when backprobed, but the PCM is still reporting low signals, disconnect both the APP sensor and PCM connectors. Use the DMM to check for continuity on the PDS2 and LPDS signal wires from the APP sensor connector to their respective pins at the PCM connector. There should be very low resistance (typically <0.5 ohms). Also, check these wires for shorts to ground or shorts to power by checking resistance between each signal wire and vehicle chassis ground, and between each signal wire and the 12V power supply.
5. PCM Pin-out Verification: If all sensor and wiring tests are good, but the code persists, it might indicate an internal PCM fault. However, this is rare and should only be considered after exhaustive testing of all other components.

Recommended Repairs and Solutions

Based on the diagnostic findings, the following repairs and solutions are recommended:

• Replace the Accelerator Pedal Position (APP) Sensor Assembly: This is the most common resolution if the DMM tests confirm the APP sensor itself is failing to produce adequate voltage signals for PDS2 and LPDS. Always use an OEM-equivalent or genuine OEM part for reliability. After replacement, clear codes and perform any necessary sensor relearn procedures specified by the vehicle manufacturer.
• Repair or Replace Wiring Harness: If visual inspection and continuity/resistance tests identify damaged or shorted wiring, repair the affected section of the harness. Use appropriate gauge wire, solder connections, and protect with heat shrink tubing. For extensive damage, replacement of the relevant harness segment may be necessary.
• Clean and Secure Electrical Connectors: If corrosion or loose terminals are found at the APP sensor or PCM connector, carefully clean the terminals using electrical contact cleaner and a small brush. Apply dielectric grease to prevent future corrosion and ensure a secure connection.
• PCM Replacement/Reprogramming: If all APP sensor and wiring tests unequivocally point to a PCM fault, the PCM may need to be replaced or reprogrammed. This is a costly and complex repair, usually requiring specialized tools and programming, and should only be performed as a last resort after all other possibilities have been ruled out.

Mechanics’ Tips:

• Always start with the simplest checks, such as visual inspection and live data, before proceeding to more invasive DMM testing.
• Ensure the vehicle’s battery voltage is stable and fully charged during diagnostics, as low voltage can sometimes cause erratic sensor readings.
• After any repair, clear the trouble codes and perform a thorough test drive, monitoring the APP sensor PDS2 and LPDS PIDs with the scanner to confirm the voltages are now within the expected range and correlating correctly across the pedal’s full travel.
• Consult manufacturer-specific service information for exact wiring diagrams, pin-out locations, and expected voltage ranges, as these can vary significantly between make and model.