P1574

What Does Code P1574 Mean?

DTC P1574 indicates a detected disagreement or inconsistency between the primary and secondary Throttle Position Sensors (TPS), often designated as TPS1 and TPS2, or TPS ‘A’ and TPS ‘B’. In modern drive-by-wire Electronic Throttle Control (ETC) systems, the Engine Control Module (ECM), also known as the Powertrain Control Module (PCM), relies on redundant TPS sensors to accurately monitor the throttle plate’s actual position. These sensors typically provide a variable voltage signal directly proportional to the throttle plate’s angle. For instance, at idle, TPS1 might output approximately 0.5V, while TPS2 outputs around 4.5V (or vice-versa, or both may show increasing voltage from a low base, but with an inverse relationship or a specific offset). The ECM continuously monitors the correlation between these two signals. When the voltage readings from these two sensors deviate beyond a manufacturer-specified calibrated threshold for a sustained period, the ECM interprets this as a critical failure in throttle position rationality, triggering P1574. This fault can indicate a potential loss of precise throttle control, leading the ECM to engage a “limp-home” mode, restricting engine power and throttle response to prevent unintended acceleration or deceleration.

Common Symptoms

• Malfunction Indicator Lamp (MIL) illumination on the dashboard.
• Reduced engine power or activation of “limp-home” mode, severely limiting vehicle speed and acceleration.
• Erratic or unstable idle speed, sometimes fluctuating significantly.
• Poor throttle response or a “dead pedal” sensation where pressing the accelerator has little to no effect.
• Hesitation or stalling during acceleration or deceleration.
• In some severe cases, a no-start condition if the ECM cannot establish a safe throttle position.
• The engine may enter a fail-safe mode, preventing normal throttle operation.

What Causes the Code P1574?

• Faulty Electronic Throttle Body (ETB) Assembly: The TPS sensors are often integrated into the ETB. Internal wear, contamination, or electrical failure within the ETB can cause sensor disagreement.
• Internal Throttle Position Sensor (TPS) Fault: One or both of the redundant TPS sensors within the ETB may be providing inaccurate or intermittent signals due to wear on the resistive track or internal circuit failure.
• Wiring Harness Issues: An open circuit, short to voltage, short to ground, or high resistance in the wiring harness connecting the ETB to the ECM can disrupt sensor signal integrity. This includes corroded terminals at the connector.
• Contamination or Mechanical Sticking: Carbon buildup around the throttle plate or an obstruction within the throttle body bore can impede smooth throttle plate movement, causing a physical disagreement between the actual throttle angle and the reported sensor values.
• ECM/PCM Failure: While less common, an internal fault within the ECM/PCM affecting its ability to correctly process TPS signals or provide reference voltages can trigger this code.

How to Diagnose and Troubleshoot

A systematic diagnostic approach is crucial for P1574. Begin by ensuring battery voltage is stable and sufficient (typically 12.6V or higher).

1. OBD-II Scanner Initial Check: Connect an advanced OBD-II scanner to confirm P1574 and check for any related or accompanying codes (e.g., P2135, P0121, P0221). Record freeze frame data, as this provides valuable information on engine conditions when the fault occurred. Clear codes after documentation.
2. Visual Inspection: Perform a thorough visual inspection of the entire Electronic Throttle Body (ETB) assembly, its electrical connector, and the associated wiring harness leading to the ECM. Look for signs of physical damage, corrosion, frayed wires, bent pins, or loose connections. Inspect the throttle plate for excessive carbon buildup or foreign objects that might impede its movement.
3. Live Data Monitoring (Key On, Engine Off – KOEO): With the ignition on and engine off, monitor the live data streams for the primary (TPS1/TP Sensor A) and secondary (TPS2/TP Sensor B) throttle position sensor voltages or percentages.

• Observe their readings at the fully closed (idle) position. They should show a specific, often inverse, correlation or a consistent offset (e.g., 0.5V and 4.5V, or 15% and 85%).
• Slowly depress the accelerator pedal through its full range while observing both sensor readings. They should move smoothly and synchronously, maintaining their relative correlation. Look for any sudden drops, spikes, flatlining, or significant deviations between the two sensor values.

Live Data Monitoring (Engine Running – KOER): Start the engine and repeat the live data monitoring of TPS1 and TPS2. Observe idle readings and then slowly depress the accelerator. Any discrepancies noted in KOEO should be more pronounced under engine load.

Circuit Integrity Test (DMM): If live data shows inconsistency, a digital multimeter (DMM) can be used to test the circuits.

• Power and Ground: Backprobe the TPS connector to verify the 5-volt reference voltage and a good ground signal are present for both sensors from the ECM.
• Signal Wires: If accessible, backprobe the signal wires for both TPS sensors at the ETB connector. Manually operate the throttle plate while observing the DMM readings. Compare the voltage sweep of each sensor. Any intermittent readings or lack of voltage change on one sensor indicates an internal sensor or wiring issue.
• Wiggle Test: With the engine running and live data active, gently wiggle the wiring harness and connectors for the ETB and ECM. Observe if this action causes any fluctuations or dropouts in the TPS sensor readings, indicating an intermittent connection.

Throttle Body Operation Check: If accessible, gently attempt to move the throttle plate manually (engine off). It should move smoothly without binding. A sticking throttle plate can cause sensor disagreement.

Component Swap (if applicable and safe): If a known good ETB is available, temporarily swapping it can quickly rule out the ETB as the source of the problem.

Recommended Repairs and Solutions

Based on the diagnostic findings, the following repairs are typically recommended:

1. Electronic Throttle Body (ETB) Replacement: This is the most common solution for P1574, as the TPS sensors are integral to the ETB on most modern vehicles and cannot be serviced separately. Ensure the replacement ETB is an OEM part or a high-quality aftermarket unit to ensure proper functionality and calibration with the ECM.
2. Wiring Harness Repair/Replacement: If the diagnosis indicates a fault in the wiring harness (open, short, or high resistance), repair the damaged section of the wiring or replace the affected portion of the harness. Ensure all connections are clean, tight, and free of corrosion.
3. Throttle Body Cleaning: If excessive carbon buildup is identified as the cause of sticking, clean the throttle body using a specialized throttle body cleaner (ensure it’s safe for coated throttle bodies). Be gentle and avoid manually forcing the throttle plate open on drive-by-wire systems to prevent damage to internal gears or sensors. After cleaning, inspect for smooth operation.
4. ECM/PCM Replacement/Reprogramming: Only consider this as a last resort if all other components and wiring have been thoroughly tested and verified to be in good working order. A faulty ECM is a rare cause for P1574. If replaced, the new ECM will require programming or flashing to the vehicle’s specifications.

Important Mechanics’ Tips: After replacing the ETB, it is often necessary to perform a “throttle body relearn” procedure using a capable scan tool. This procedure allows the ECM to learn the new throttle body’s idle position and full range, ensuring correct operation. Always clear all diagnostic trouble codes after completing any repair and perform a comprehensive road test to confirm the fault has been resolved and the code does not return.