Introduction:
The diagnostic trouble code (DTC) P1579 indicates a specific issue within a vehicle's powertrain control module (PCM). It signifies that the PCM has detected an unexpected transition from Park/Neutral (P/N) to Drive/Reverse (D/R) while the throttle angle is significantly high, triggering a power reduction mode. This is often a safety mechanism designed to prevent potential damage to the engine or transmission.
Understanding P1579: A Comprehensive Guide
| Topic | Description | Possible Causes |
|---|---|---|
| Definition | P1579 - P/N to D/R at High Throttle Angle - Power Reduction Mode Active. Indicates an issue where the PCM detects shifting from Park/Neutral to Drive/Reverse with excessive throttle input. | N/A |
| Symptoms | Reduced engine power, limp mode activation, illuminated check engine light, harsh shifting, potential stalling, difficulty accelerating. | N/A |
| PCM Operation & Logic | The PCM monitors throttle position, transmission gear selector position, and vehicle speed. It expects a gradual increase in throttle angle after the transmission is engaged. P1579 is triggered when the PCM detects high throttle during or before gear engagement. | Defective throttle position sensor (TPS), faulty transmission range sensor (TRS), wiring harness issues (shorts, opens, corrosion), PCM malfunction, incorrect idle speed adjustment, vacuum leaks affecting engine load, mechanical transmission problems, incorrect or contaminated transmission fluid, faulty shift solenoid, Torque converter issues. |
| Throttle Position Sensor (TPS) | The TPS provides the PCM with information about the driver's intended acceleration. An inaccurate or erratic TPS signal can lead to misinterpretation of throttle angle, triggering the P1579 code. | Damaged TPS, loose connection, shorted or open circuit in TPS wiring, corrosion in TPS connector, incorrect TPS adjustment. |
| Transmission Range Sensor (TRS) | The TRS (also known as neutral safety switch or inhibitor switch) informs the PCM about the gear selector's position (P, R, N, D, etc.). A faulty TRS can send incorrect signals, confusing the PCM about the transmission's state. | Misalignment of TRS, damaged TRS, loose connection, shorted or open circuit in TRS wiring, corrosion in TRS connector, mechanical failure within the TRS. |
| Wiring Harness Issues | Damaged, corroded, or shorted wiring connecting the TPS, TRS, and PCM can disrupt communication and cause the PCM to misinterpret sensor data. | Rodent damage, chafing against engine components, exposure to extreme temperatures, corrosion due to water intrusion, loose or damaged connectors. |
| PCM Malfunction | Although less common, the PCM itself can be faulty, leading to incorrect processing of sensor data and triggering the P1579 code. | Internal PCM failure, damage due to voltage spikes, software corruption. |
| Idle Speed Adjustment | An improperly adjusted idle speed can cause the engine to rev higher than expected, especially when shifting from Park/Neutral to Drive/Reverse. This can be misinterpreted by the PCM as high throttle. | Incorrect idle speed setting, vacuum leaks affecting idle speed, faulty idle air control (IAC) valve. |
| Vacuum Leaks | Vacuum leaks can disrupt the engine's air-fuel mixture, causing it to run lean and potentially increase engine speed. This can create a situation where the PCM thinks the throttle is higher than it actually is. | Cracked or disconnected vacuum hoses, faulty intake manifold gasket, damaged throttle body gasket. |
| Mechanical Transmission Issues | Internal transmission problems such as worn clutches or bands can cause slippage and erratic behavior, leading the PCM to activate power reduction mode. | Worn or damaged clutches, bands, or seals, valve body issues, internal leaks. |
| Transmission Fluid | Low, contaminated, or incorrect transmission fluid can cause poor shifting performance and contribute to the P1579 code. | Low fluid level, contaminated fluid, incorrect fluid type. |
| Shift Solenoids | Shift solenoids control the flow of transmission fluid to engage different gears. A faulty solenoid can cause erratic shifting and trigger the P1579 code. | Faulty solenoid, wiring issues to the solenoid. |
| Torque Converter | The torque converter transfers power from the engine to the transmission. Issues with the torque converter, such as a failing stator or lock-up clutch, can cause power loss and trigger the P1579 code. | Damaged stator, failing lock-up clutch. |
| Diagnostic Steps | Read and record all DTCs, inspect wiring and connectors, check TPS signal, test TRS, check for vacuum leaks, check transmission fluid level and condition, monitor live data, perform transmission tests. | N/A |
| Possible Solutions | Replace faulty TPS, replace faulty TRS, repair wiring harness, replace faulty PCM (rare), adjust idle speed, repair vacuum leaks, repair or replace transmission, replace transmission fluid, replace shift solenoid, replace torque converter. | N/A |
Detailed Explanations
Definition: P1579 - P/N to D/R at High Throttle Angle - Power Reduction Mode Active. This DTC indicates that the vehicle's computer system (PCM) has detected a situation where the transmission is being shifted from Park or Neutral into Drive or Reverse while the engine throttle is open more than expected. This triggers a safety feature, limiting engine power to prevent potential damage.
Symptoms: The most noticeable symptom of a P1579 code is reduced engine power. The vehicle may enter limp mode, restricting speed and acceleration. The check engine light will illuminate. You may also experience harsh shifting or even stalling. The car might have difficulty accelerating from a stop.
PCM Operation & Logic: The PCM constantly monitors various sensors, including the TPS, TRS, and vehicle speed sensor. The PCM anticipates a specific sequence of events: the gear selector moving to Drive or Reverse followed by a gradual increase in throttle. If the PCM sees a high throttle percentage simultaneously with or before the engagement of Drive or Reverse, it interprets this as an abnormal condition. This triggers the P1579 code and activates power reduction to protect the powertrain.
Throttle Position Sensor (TPS): The TPS is a potentiometer that measures the position of the throttle plate. It sends a voltage signal to the PCM, indicating how far the driver has pressed the accelerator pedal. A faulty TPS can provide an inaccurate or fluctuating signal, leading the PCM to believe the throttle is open wider than it actually is. This inaccurate data can trigger the P1579 code.
Transmission Range Sensor (TRS): The TRS, also referred to as the neutral safety switch or inhibitor switch, is located on the transmission. It informs the PCM about the position of the gear selector (P, R, N, D, etc.). A malfunctioning TRS can send incorrect or intermittent signals, causing the PCM to misinterpret the transmission's state. For example, it might signal that the transmission is in Drive when it's actually in Neutral, or vice-versa.
Wiring Harness Issues: The wiring harness connects all the sensors and components to the PCM. Damage to the wiring, such as shorts, opens, or corrosion, can disrupt the flow of information. This can lead to inaccurate sensor readings and trigger the P1579 code. Inspecting the wiring harness for any signs of damage is crucial in diagnosing this issue.
PCM Malfunction: Although less frequent than other causes, the PCM itself can be the source of the problem. A faulty PCM may misinterpret sensor data or fail to process information correctly. However, before replacing the PCM, it's essential to rule out all other potential causes, as PCM replacement often requires reprogramming.
Idle Speed Adjustment: If the engine's idle speed is set too high, it can create a situation where the PCM perceives a high throttle angle even when the accelerator pedal is not depressed. This is particularly relevant when shifting from Park/Neutral to Drive/Reverse, as the engine's increased RPM can be misinterpreted.
Vacuum Leaks: Vacuum leaks introduce unmetered air into the engine, causing it to run lean. To compensate, the PCM may increase the idle speed, which can lead to a higher throttle angle reading. This can contribute to the P1579 code being triggered.
Mechanical Transmission Issues: Internal problems within the transmission, such as worn clutches, bands, or valve body issues, can cause slippage and erratic shifting. This can confuse the PCM and lead to the activation of the power reduction mode associated with the P1579 code.
Transmission Fluid: Maintaining the correct transmission fluid level and condition is vital for smooth operation. Low fluid levels, contaminated fluid, or using the wrong type of fluid can cause poor shifting performance and contribute to the P1579 code. Checking the fluid level and condition should be part of the diagnostic process.
Shift Solenoids: Shift solenoids are electrically operated valves that control the flow of transmission fluid to engage different gears. A malfunctioning shift solenoid can cause erratic shifting and potentially trigger the P1579 code by sending incorrect signals to the PCM or causing unexpected shifts.
Torque Converter: The torque converter acts as a fluid coupling between the engine and transmission. Issues within the torque converter, such as a failing stator or lock-up clutch, can cause power loss and lead the PCM to activate power reduction mode, resulting in the P1579 code.
Diagnostic Steps: The proper diagnostic procedure involves several steps. First, read and record all DTCs using a scan tool. Next, visually inspect the wiring and connectors associated with the TPS, TRS, and PCM. Check the TPS signal using a multimeter or scan tool to ensure it's within specifications. Test the TRS to verify it's sending the correct signals to the PCM. Check for vacuum leaks using a smoke machine or other diagnostic tools. Check the transmission fluid level and condition. Monitor live data from the TPS and TRS while shifting gears. Finally, perform transmission tests as needed.
Possible Solutions: The solution depends on the root cause of the problem. If the TPS is faulty, replace the faulty TPS. If the TRS is malfunctioning, replace the faulty TRS. If there are damaged wires, repair the wiring harness. In rare cases, the PCM may need to be replaced. Adjusting the idle speed can resolve issues related to high idle. Repairing vacuum leaks can correct air-fuel mixture imbalances. Repairing or replacing the transmission may be necessary for internal transmission problems. Replacing the transmission fluid with the correct type and amount can improve shifting performance. Replace shift solenoid if testing reveals it's faulty. Replace the torque converter if it's determined to be the cause of the issue.
Frequently Asked Questions
What does the P1579 code mean? It means the engine control unit detected an unexpected shift from Park/Neutral to Drive/Reverse while the throttle was open too much, causing a power reduction mode.
What are the most common symptoms of a P1579 code? Reduced engine power and the check engine light being illuminated are the most common symptoms.
Can a bad TPS cause a P1579 code? Yes, a faulty TPS can send inaccurate throttle position data to the PCM, triggering the code.
Is it safe to drive with a P1579 code? Driving is not recommended, as the vehicle is in power reduction mode and there's potentially an underlying mechanical issue.
How do I fix a P1579 code? The fix depends on the cause. You might need to replace the TPS or TRS, repair wiring, or address transmission issues. Professional diagnosis is recommended.
Conclusion
The P1579 code indicates a complex issue involving the interaction between the engine and transmission control systems. A thorough diagnostic process is crucial to identify the root cause and implement the appropriate repair. Addressing this issue promptly can prevent further damage and ensure the vehicle's safe and efficient operation.