P1585

What Does Code P1585 Mean?

DTC P1585 indicates a detected malfunction within the vehicle’s Throttle Control Unit, often specifically referring to issues with the Electronic Throttle Body (ETB) assembly itself, or its immediate control circuit. In modern drive-by-wire systems, the Engine Control Module (ECM) or Powertrain Control Module (PCM) directly commands the throttle plate position via an integrated DC motor within the ETB. The ETB assembly typically houses the throttle motor and multiple Throttle Position Sensors (TPS) that provide redundant feedback to the ECM/PCM regarding the actual throttle plate angle. The ECM/PCM continuously monitors this feedback and compares the actual throttle plate position with the commanded position. When the ECM/PCM detects a significant discrepancy between the commanded throttle plate angle and the actual throttle plate angle, or an internal fault within the ETB’s motor control circuit, or an inconsistency in the TPS signals that exceeds predefined calibration parameters for a specified duration, it will set DTC P1585. This code primarily affects the engine’s air intake management and can severely impact drivability, often leading to activation of a failsafe or “limp home” mode.

Common Symptoms

• Illuminated Malfunction Indicator Lamp (MIL) or “Check Engine” light.
• Reduced engine power or “Limp Home” mode activation, limiting engine RPM and vehicle speed.
• Erratic or high idle speed, or conversely, a complete inability to maintain idle.
• Poor throttle response or complete lack of response to accelerator pedal input.
• Engine stalling, particularly during deceleration or when coming to a stop.
• Unintended acceleration or deceleration (less common but possible due to erroneous throttle plate positioning).
• Hesitation or surging during acceleration.

What Causes the Code P1585?

• Faulty Electronic Throttle Body (ETB) assembly: This is the most common cause, encompassing internal motor failure, worn gears, damaged throttle plate, or integrated Throttle Position Sensor (TPS) malfunction.
• Damaged or corroded wiring harness: Open circuits, shorts, or excessive resistance in the wiring between the ETB and the ECM/PCM.
• Poor electrical connections: Loose, corroded, or damaged pins at the ETB connector or the ECM/PCM connector.
• Contamination or obstruction: Excessive carbon buildup or foreign debris physically impeding the throttle plate’s movement.
• ECM/PCM malfunction: A rare but possible failure of the ECM/PCM’s internal ETB driver circuit or processing unit.
• Insufficient voltage supply to the ETB: Although often accompanied by other voltage-related codes, a poor power supply can cause the ETB to malfunction.

How to Diagnose and Troubleshoot

Diagnosis of P1585 requires a systematic approach, utilizing an OBD-II scanner and a digital multimeter (DMM).

1. Perform a Thorough Visual Inspection:
   • Inspect the Electronic Throttle Body (ETB) for obvious physical damage, signs of overheating, or excessive carbon buildup around the throttle plate.
   • Check the entire wiring harness connecting the ETB to the ECM/PCM for any signs of chafing, cuts, corrosion, or pinches. Pay close attention to areas where the harness passes through firewalls or near hot engine components.
   • Inspect the ETB electrical connector and the corresponding ECM/PCM connector for bent, corroded, or pushed-out pins. Ensure secure connections.
2. Scan for Diagnostic Trouble Codes (DTCs) and Freeze Frame Data:
   • Connect an OBD-II scanner and retrieve all stored DTCs. Note any other related codes, especially those pertaining to Accelerator Pedal Position (APP) sensors, TPS, or voltage supply, as they may provide additional clues.
   • Analyze freeze frame data to understand the engine operating conditions (engine speed, load, coolant temperature, vehicle speed, etc.) at the moment P1585 was set. This can help replicate the fault.
3. Monitor Live Data with an OBD-II Scanner:
   • With the ignition ON (engine OFF) and then with the engine running, monitor the following live data parameters:
     • Throttle Position Sensor (TPS) 1 and TPS 2 voltages/percentages: Observe for smooth, correlative changes as the accelerator pedal is slowly pressed and released. Look for sudden drops, spikes, or flat spots, which indicate an internal sensor fault within the ETB. The two sensors should typically read opposite or inverse values but change proportionally (e.g., one increases from 0.5V to 4.5V while the other decreases from 4.5V to 0.5V, or both increase/decrease proportionally but with differing base voltages).
     • Accelerator Pedal Position Sensor (APP) 1 and APP 2 voltages/percentages: Ensure these signals are smooth and correlate with pedal input. While P1585 points to the ETB, APP sensor issues can indirectly cause the ECM to misinterpret throttle demands.
     • Desired Throttle Angle vs. Actual Throttle Angle: Compare these two parameters. A significant and persistent deviation is a direct indicator of the P1585 condition.
     • ETB Motor Command (or similar parameter): This indicates the ECM’s duty cycle or voltage command to the ETB motor. Observe how it correlates with desired and actual angles.
   • Perform any available ETB functional tests or bidirectional controls using the scanner to cycle the throttle plate and observe its response and sensor feedback.
4. Perform Electrical Circuit Checks with a DMM:
   • Power and Ground Supply to ETB: With the ignition ON, back-probe the ETB connector to verify proper 12V power supply (refer to service manual for specific pins) and a solid ground connection. A weak power or ground can cause intermittent operation.
   • Wiring Integrity (Continuity and Resistance): Disconnect both the ETB and ECM/PCM connectors. Use the DMM to check for continuity on all control wires between the two connectors. Check for shorts to ground and shorts to voltage on each wire. High resistance or intermittent continuity indicates a wiring harness issue.
   • ETB Motor Resistance (if applicable): Some service manuals provide resistance specifications for the ETB motor. If accessible, measure the resistance across the motor terminals; however, this is often not possible without disassembling the ETB.
5. Check for Mechanical Binding: With the ignition OFF, gently attempt to push the throttle plate open and closed by hand. It should move smoothly without excessive force or binding. If it binds, physically clean or replace the ETB.
6. ECM/PCM Diagnosis (Last Resort): If the ETB, wiring, and connectors all test good according to specifications, and the issue persists, the ECM/PCM may be at fault. This typically requires specialized diagnostic equipment and should only be considered after ruling out all other components.

Recommended Repairs and Solutions

Once the diagnostic steps have pinpointed the root cause, the following repairs are typically recommended:

• Replace the Electronic Throttle Body (ETB) Assembly: This is by far the most common repair for P1585, as the internal components (motor, gears, TPS) are often integrated and not individually serviceable. Ensure that a new or remanufactured ETB is sourced from a reputable OEM or quality aftermarket supplier.
• Repair or Replace Damaged Wiring and Connectors: If the visual inspection or DMM tests reveal damaged, corroded, or open/shorted wiring, the affected sections of the harness should be repaired using proper automotive wiring techniques (soldering and heat shrink) or replaced entirely. Similarly, damaged connectors should be replaced.
• Clean the Throttle Body: While less likely to resolve a specific “unit malfunction” code (P1585 often implies an electrical/mechanical failure of the unit itself), heavy carbon buildup can physically impede the throttle plate’s movement. Use an appropriate throttle body cleaner and a soft brush/cloth to carefully clean the throttle plate and bore, taking care not to damage internal electronic components or force the throttle plate open.
• ECM/PCM Replacement/Reprogramming: If all other components have been thoroughly tested and verified as functional, and the ECM/PCM is definitively identified as the cause, it will require replacement and subsequent programming to the vehicle’s specific VIN and options. This is a complex and costly repair, so thorough testing of all other components is critical before pursuing this option.

Important Mechanics’ Tips:

• Battery Disconnection: Always disconnect the negative battery terminal before performing any electrical work or replacing components to prevent accidental short circuits or component damage.
• Throttle Relearn Procedure: After replacing the ETB, it is almost always necessary to perform a “throttle relearn” or “idle relearn” procedure using an advanced OBD-II scanner or factory diagnostic tool. This procedure allows the ECM/PCM to learn the new throttle body’s idle position and operational parameters. Failure to perform this step can result in rough idle, stalling, or continued drivability issues.
• Quality Parts: Invest in high-quality OEM or reputable aftermarket replacement parts to ensure longevity and proper function. Inferior components may lead to recurring issues.
• Clear Codes and Test Drive: After any repair, clear all stored DTCs from the ECM/PCM. Then, perform an extended test drive under various driving conditions (idle, light acceleration, heavy acceleration, highway speeds) to confirm the repair has resolved the issue and that P1585, along with any related codes, does not return.