P1579

What Does Code P1579 Mean?

DTC P1579, “ETC In Power Limiting Mode,” indicates that the Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected a critical fault within the Electronic Throttle Control (ETC) system and has, as a safety measure, intentionally engaged a power limiting or “limp home” mode. The ETC system, commonly known as “drive-by-wire,” replaces the traditional mechanical linkage between the accelerator pedal and the throttle body with electronic sensors and actuators. It comprises an Accelerator Pedal Position (APP) sensor, an Electronic Throttle Body (ETB) assembly (which includes a throttle plate, a DC motor actuator, and integral Throttle Position Sensors – TPS), and the ECM/PCM. When the ECM detects a significant discrepancy between the commanded throttle position (based on APP sensor input) and the actual throttle position (based on TPS feedback), or an internal fault within the ETB, it determines that safe and precise throttle control cannot be maintained. To prevent unintended acceleration or engine damage, the ECM limits engine power, often by restricting throttle plate opening, reducing fuel delivery, and/or limiting engine RPM, while illuminating the Malfunction Indicator Lamp (MIL).

Common Symptoms

• Reduced Engine Power: The most prominent symptom is a noticeable and significant reduction in engine power and acceleration, often referred to as “limp mode.”
• Limited Engine RPM: The engine’s maximum rotational speed (RPM) may be severely restricted, often to 2,500-3,500 RPM.
• Limited Vehicle Speed: The vehicle’s top speed will be restricted, making it difficult to maintain highway speeds.
• Poor Throttle Response: The accelerator pedal may feel unresponsive or have a delayed reaction to input.
• Illuminated Malfunction Indicator Lamp (MIL): The “Check Engine” light will be on.
• Hesitation or Surge: In some cases, erratic throttle behavior, such as hesitation during acceleration or an unexpected surge, may precede or accompany the limp mode.

What Causes the Code P1579?

• Faulty Electronic Throttle Body (ETB) Assembly: This is a primary suspect. Internal failures can include a malfunctioning throttle actuator motor, worn gears, a contaminated throttle plate causing sticking, or faulty Throttle Position Sensors (TPS) integrated within the ETB.
• Defective Accelerator Pedal Position (APP) Sensor: A common cause is an APP sensor that provides erratic, incorrect, or no signal. This often involves a discrepancy between the multiple internal potentiometers (typically two or three) within the sensor.
• Wiring Harness Issues: Open circuits, short circuits to voltage or ground, or high resistance in the wiring between the APP sensor, the ETB, and the ECM/PCM. Corrosion at connector terminals is also a frequent culprit.
• ECM/PCM Internal Failure: While less common, an internal fault within the ECM/PCM responsible for processing ETC signals or controlling the throttle actuator can trigger this code. This is usually considered after all other components are thoroughly ruled out.
• Insufficient Power or Ground Supply: A loss of proper voltage supply or a high-resistance ground path to either the APP sensor or the ETB can cause erratic operation and trigger the ETC into power limiting mode.

How to Diagnose and Troubleshoot

Diagnosing P1579 requires a methodical approach, focusing on the integrity of the ETC system’s electrical signals and mechanical operation.

1. Initial Scan Tool Analysis:
   • Connect an OBD-II scanner and confirm the presence of P1579.
   • Check for any other related ETC codes (e.g., P2100-P2110 for throttle actuator control, P0120-P0124 for TPS, P0220-P0229 for TPS/APP sensor performance). The presence of other codes can provide a more specific diagnostic direction.
   • Review Freeze Frame Data to understand the engine operating conditions (RPM, engine load, vehicle speed, throttle position) when the fault was first set. This can offer clues about the triggering event.
2. Visual Inspection:
   • Carefully inspect the wiring harness for the APP sensor and the ETB. Look for signs of physical damage, chafing, bare wires, corrosion at connectors, or improper repairs.
   • Check the throttle body for excessive carbon buildup around the throttle plate and bore. Significant carbon accumulation can cause the plate to stick or operate sluggishly. Ensure the throttle plate moves freely by hand when the engine is off (and typically with the battery disconnected to prevent unexpected actuation).
   • Verify all electrical connectors are securely seated and free from corrosion.
3. Live Data Monitoring (with OBD-II Scanner):
   • With the engine off, key on, monitor APP Sensor 1 & 2 (and 3, if applicable) voltages. Slowly depress the accelerator pedal from 0% to 100%. The voltages should increase smoothly and proportionally, maintaining a specific ratio (e.g., APP1 often double APP2). Look for sudden drops, spikes, or flat spots in the readings.
   • Similarly, monitor Throttle Position Sensor 1 & 2 voltages from the ETB. These should also change smoothly as the throttle plate is commanded to open and close (or manually manipulated if safe and feasible). Again, look for signal dropouts or erratic readings.
   • Monitor the “Desired Throttle Angle” and “Actual Throttle Angle” PIDs. A significant and persistent discrepancy between these two values is a strong indicator of an ETC system malfunction.
4. Digital Multimeter (DMM) Testing:
   • APP Sensor Voltage Supply and Ground: Disconnect the APP sensor connector. Using a DMM, check for the specified 5V reference voltage and a good ground at the appropriate pins of the harness side connector.
   • APP Sensor Signal Integrity (Back-Probing): Reconnect the APP sensor. With the key on, back-probe the signal wires of the APP sensor. Slowly depress the pedal while observing the voltage readings on the DMM for each sensor circuit. The voltage should rise smoothly without interruption.
   • ETB Voltage Supply and Ground: Disconnect the ETB connector. Check for battery voltage supply (for the motor) and 5V reference voltage (for TPS) and proper ground at the harness side.
   • ETB TPS Signal Integrity (Back-Probing): Reconnect the ETB. Back-probe the TPS signal wires. Observe voltage changes as the throttle plate is manually moved (if possible) or commanded via a bi-directional scan tool.
   • Wiring Continuity and Resistance: With the battery disconnected and both ECM/PCM and component connectors unplugged, test for continuity and resistance in all relevant wires between the ECM/PCM, APP sensor, and ETB. Look for opens (infinite resistance), shorts to ground (0 ohms to ground), shorts to power, or excessive resistance (>1-2 ohms).
5. Bi-directional Scan Tool Tests:
   • If available, use a bi-directional scan tool to perform an “ETC Actuator Test” or “Throttle Body Relearn.” This commands the throttle plate to open and close, allowing you to observe its physical movement and the feedback from the TPS sensors in real-time.

Recommended Repairs and Solutions

Once the root cause has been identified through systematic diagnosis, the following repairs are typically recommended:

1. Clean the Electronic Throttle Body: If significant carbon buildup is observed, carefully clean the throttle plate and bore using a throttle body specific cleaner and a lint-free cloth. Avoid harsh scrubbing or abrasive tools, as this can damage the special coating on the throttle plate or bore. Never force the throttle plate open manually on some ETC systems, as it can damage the internal gearing.
2. Replace the Accelerator Pedal Position (APP) Sensor: If the APP sensor’s signal output is erratic or incorrect, replacement is necessary. Ensure you obtain an OEM-quality replacement. After replacement, some vehicles may require an APP sensor relearn procedure using a scan tool.
3. Replace the Electronic Throttle Body (ETB) Assembly: If the throttle actuator motor, gears, or internal TPS within the ETB are found to be faulty, the entire throttle body assembly typically needs to be replaced. Manufacturers usually do not offer individual components for repair. After replacement, a throttle body relearn or calibration procedure is almost always required using a capable scan tool to ensure the ECM correctly maps the new throttle body’s position sensors.
4. Repair or Replace Wiring Harness: If damaged or high-resistance wiring is identified, repair the affected section using proper wiring repair techniques (e.g., soldering and heat-shrink tubing) or replace the entire harness segment if damage is extensive. Ensure all connectors are clean, tight, and corrosion-free.
5. ECM/PCM Replacement/Reprogramming: Only consider ECM/PCM replacement as a last resort, after all other components and wiring have been thoroughly tested and confirmed to be in good working order. If the ECM is replaced, it will require specific programming and potentially immobilizer synchronization to the vehicle’s VIN. In some cases, a software update (reprogramming) to the existing ECM might resolve an intermittent issue if a known technical service bulletin (TSB) exists.

Mechanic’s Tip: Always clear the DTCs after performing a repair. Then, conduct an extended test drive under various driving conditions, including varying throttle inputs and speeds, to confirm the repair and ensure the code does not return. Monitor live data during the test drive to verify smooth and consistent operation of both APP and TPS sensors.