What Does Code P1120 Mean?
DTC P1120 signifies a condition where the Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected that the Throttle Position Sensor (TPS) signal voltage is outside its predefined operational range. This is often a manufacturer-specific code, predominantly found in Toyota/Lexus vehicles, indicating a fundamental fault in the TPS signal integrity rather than just an inconsistency. The TPS is a crucial input sensor that relays the throttle plate’s angular position to the ECM, which then utilizes this data to calculate appropriate fuel injection quantity, ignition timing, transmission shift points, and idle speed control strategies. When the ECM detects a voltage that is either critically too low (e.g., 0V or significantly below the expected idle voltage) or critically too high (e.g., 5V or significantly above the expected Wide Open Throttle voltage) for the current operating conditions, it interprets this as an “out of range” condition, triggering the P1120 code. This signal deviation prevents the ECM from accurately determining engine load and air intake, severely impacting engine performance and control.
Common Symptoms
- Malfunction Indicator Lamp (MIL) Illumination: The “Check Engine” light will be illuminated on the dashboard.
- Rough or Erratic Idle: The engine may idle unstably, fluctuate in RPM, or stall frequently.
- Poor Acceleration and Hesitation: The vehicle may experience a noticeable lack of power during acceleration or a delay in response to throttle input.
- Engine Surging or Bucking: The vehicle might experience sudden increases or decreases in engine speed without driver input, especially at steady throttle.
- Reduced Engine Power (Limp Home Mode): The ECM may limit engine power and vehicle speed to prevent damage due to the uncertain throttle position.
- Transmission Shifting Irregularities: Incorrect throttle position data can lead to harsh, delayed, or improper transmission shifts.
- Difficulty Starting: While less common, an ECM unable to ascertain throttle position may struggle to initiate proper starting procedures.
What Causes the Code P1120?
- Faulty Throttle Position Sensor (TPS): The most common cause, involving internal electrical failure, wear on the resistive track, or mechanical degradation of the sensor itself.
- Wiring Harness Issues:
- Open circuit: A break in the TPS signal wire, 5V reference wire, or ground wire.
- Short circuit: A direct connection to vehicle voltage (e.g., 12V) or ground in any of the TPS circuit wires.
- Corrosion or damage: Degradation of pins within the TPS connector or the main engine harness leading to the TPS.
- ECM/PCM Failure: Although rare, an internal malfunction within the ECM/PCM preventing it from supplying the correct 5V reference voltage or accurately interpreting the TPS signal can trigger this code.
- Throttle Body Mechanical Malfunction: A physically binding or sticky throttle plate preventing the TPS from achieving its true closed or wide-open positions, or a dirty throttle bore interfering with plate movement.
- 5-Volt Reference Voltage Issue: Problems with the ECM’s internal 5-volt reference circuit, which supplies power to the TPS (and often other sensors), causing an incorrect voltage supply to the sensor.
How to Diagnose and Troubleshoot
Diagnosis of P1120 requires a methodical approach using an OBD-II scanner and a Digital Multimeter (DMM).
- Initial OBD-II Scanner Diagnostics:
- Connect an OBD-II scan tool and confirm the presence of P1120. Check for any other related throttle position sensor codes (e.g., P0120, P0121, P0122, P0123) that might offer additional clues.
- Access the live data stream. Monitor the Throttle Position Sensor (TPS) percentage and TPS voltage (often labeled TP SENSOR V or similar) with the ignition ON, engine OFF (KOEO).
- At closed throttle, observe the TPS voltage. It should typically be within 0.4V to 0.9V, though exact specifications vary by manufacturer (consult service manual).
- Slowly and steadily open the throttle by hand to Wide Open Throttle (WOT). The voltage should smoothly increase from the closed throttle value to approximately 4.5V to 4.9V without any sudden drops, spikes, or flat spots in the voltage curve.
- If the voltage is consistently out of the specified range at idle or WOT, or if the sweep is erratic, the TPS or its circuit is highly suspect.
- Visual Inspection:
- Perform a thorough visual inspection of the TPS sensor and its entire wiring harness. Look for signs of physical damage, corrosion at the connector, loose pins, frayed or pinched wires, or evidence of rodent damage.
- Verify that the throttle plate within the throttle body moves freely and smoothly without any binding or sticking throughout its entire range of motion.
- Digital Multimeter (DMM) Testing at TPS Connector:
- Key OFF: Disconnect the TPS electrical connector.
- Using a wiring diagram, identify the 5V reference wire, the signal return wire, and the ground wire at the harness side of the connector.
- Key ON, Engine OFF (KOEO):
- Measure the voltage between the 5V reference pin and the ground pin on the harness side of the connector. It should be approximately 5V. If it’s significantly off (e.g., 0V or 12V), troubleshoot the ECM’s 5V reference circuit or the ground circuit.
- If the 5V reference and ground are good, reconnect the TPS.
- Key ON, Engine OFF (KOEO) – Backprobing: Carefully backprobe the TPS signal wire at the sensor connector (without damaging the wire insulation).
- Measure the voltage on the signal wire while slowly operating the throttle plate from closed to WOT. Confirm that the voltage sweep is smooth, consistent, and falls within the expected range, matching the live data observed earlier. Any abrupt changes, dead spots, or persistent out-of-range readings directly point to a faulty TPS.
- For some TPS types, a resistance test across the sensor’s pins (sensor disconnected) might be specified by the manufacturer, allowing for internal circuit checks.
- Intermittent Fault Test: While monitoring the live data stream or DMM voltage reading, gently wiggle the TPS connector and the associated wiring harness. Observe for any fluctuations or dropouts in the TPS signal voltage, which could indicate an intermittent open or short circuit.
Recommended Repairs and Solutions
Addressing the P1120 code typically involves one of the following repairs:
- Replace the Throttle Position Sensor (TPS): This is the most common and often the primary solution. Ensure the replacement TPS is an OEM part or a high-quality aftermarket equivalent. After installation, verify proper function with the scan tool. Some TPS units require a specific calibration or relearn procedure using a scan tool to set the idle position or sweep range; consult the vehicle’s service manual.
- Repair or Replace Damaged Wiring/Connector: If diagnostic steps confirm an open, short, or corroded wiring harness or connector, the affected section must be meticulously repaired using proper soldering, heat shrink, and original-style connectors. Avoid temporary splices that can fail later.
- Throttle Body Cleaning or Replacement: If a sticking or binding throttle plate is identified as the root cause, thoroughly clean the throttle body bore and plate with a suitable throttle body cleaner. If mechanical damage or excessive wear prevents smooth operation, the entire throttle body may need replacement.
- ECM/PCM Replacement: Only consider ECM/PCM replacement as a last resort, after all other components and wiring have been thoroughly tested and confirmed to be in perfect working order. ECM failure leading to P1120 is rare but possible if it cannot supply the correct 5V reference or process the TPS signal correctly. This is an expensive repair and often requires programming the new module to the vehicle.
- Clear Codes and Relearn Procedures: After any repairs, clear the P1120 code using the OBD-II scanner. Perform any necessary throttle body relearn or idle relearn procedures as specified by the manufacturer. Follow with a comprehensive test drive to ensure the code does not return and that normal engine operation is restored.
