P1520

What Does Code P1520 Mean?

DTC P1520 signifies an Intake Manifold Runner Control (IMRC) Circuit Malfunction. This code is triggered by the Engine Control Module (ECM) or Powertrain Control Module (PCM) when it detects an electrical fault within the control circuit responsible for operating the intake manifold runner control system. The IMRC system is designed to optimize airflow into the combustion chambers across varying engine speeds and loads. It typically consists of a set of flaps or valves within the intake manifold, an actuator (either electric motor or vacuum-operated solenoid), and sometimes a position sensor to provide feedback to the ECM. At lower RPMs, the IMRC system closes some runners to increase air velocity and turbulence, promoting better fuel atomization and torque. At higher RPMs, these runners open to maximize airflow for peak power. The ECM monitors the voltage, resistance, or current draw in the IMRC actuator circuit. When the ECM detects an open circuit, a short to voltage, a short to ground, or an implausible signal from the IMRC position sensor (if equipped) that does not correlate with its commanded position, it identifies a circuit malfunction and sets code P1520. This indicates an electrical integrity issue preventing the ECM from effectively controlling or monitoring the IMRC system.

Common Symptoms

• Illumination of the Check Engine Light (CEL) or Malfunction Indicator Lamp (MIL).
• Reduced engine performance, particularly noticeable at specific RPM ranges (e.g., lack of low-end torque or high-end power).
• Rough or erratic idle quality.
• Poor fuel economy.
• Engine hesitation, stumbling, or surging during acceleration.
• Failure of an emissions inspection due to increased hydrocarbon (HC) or nitrogen oxide (NOx) emissions.

What Causes the Code P1520?

• Faulty IMRC Actuator: The electric motor or vacuum solenoid responsible for moving the runner flaps may be internally shorted, open, or mechanically seized, causing excessive current draw or no circuit continuity.
• Damaged Wiring or Connectors: Frayed, corroded, or broken wiring in the IMRC control circuit from the PCM to the actuator or position sensor. Loose or contaminated connector pins.
• Faulty IMRC Position Sensor: If equipped, the sensor providing feedback to the PCM may be sending an incorrect voltage signal, indicating a circuit malfunction rather than a mechanical issue (though it might be a mechanical issue causing the sensor to report incorrectly).
• Vacuum Leaks (for vacuum-actuated systems): A compromised vacuum line, check valve, or diaphragm on a vacuum-operated IMRC actuator can prevent proper operation, leading to an electrical fault if the PCM expects a change in position that doesn’t occur.
• Carbon Buildup or Mechanical Binding: Excessive carbon deposits within the intake manifold can cause the IMRC flaps or runners to stick, leading to the actuator drawing too much current or failing to reach its commanded position, which the PCM interprets as a circuit fault.
• Faulty Powertrain Control Module (PCM): While less common, an internal fault within the PCM affecting the IMRC control driver circuit can cause this code.

How to Diagnose and Troubleshoot

Diagnosis of P1520 requires a methodical approach, focusing on the electrical integrity of the IMRC system:

1. Retrieve and Analyze DTCs and Freeze Frame Data: Use an OBD-II scanner to confirm P1520 and check for any related or accompanying codes. Analyze freeze frame data to understand engine conditions (RPM, load, temperature) when the fault was registered, which can provide clues about when the malfunction occurs.
2. Visual Inspection:
   • Inspect the IMRC actuator, its wiring harness, and connectors for any visible damage, corrosion, looseness, or signs of rodent damage.
   • For vacuum-actuated systems, inspect all vacuum lines, reservoirs, and the IMRC vacuum actuator diaphragm for cracks, leaks, or disconnections.
   • Manually try to move the IMRC flaps (if accessible) to check for binding or excessive carbon buildup. Note if movement feels rough or restricted.
3. Electrical System Checks (Using a Digital Multimeter – DMM):
   • Power Supply Check: With the ignition on, engine off (KOEO), backprobe the IMRC actuator connector to check for proper voltage supply (typically battery voltage or 5V reference) to the actuator. Also, check for a good ground connection.
   • Actuator Resistance Test: Disconnect the IMRC actuator and measure its internal resistance using a DMM. Compare this reading to manufacturer specifications. An open circuit (OL) or a resistance significantly outside specification indicates an internal fault in the actuator.
   • Continuity and Short Circuit Check: Disconnect the PCM and the IMRC actuator/sensor. Test for continuity between the PCM connector pins and the IMRC actuator/sensor connector pins to ensure no open circuits in the wiring. Also, check for short circuits to ground or to power in all wires of the IMRC circuit.
   • Position Sensor Verification (if applicable): If a separate IMRC position sensor is present, backprobe its signal wire with the KOEO. Manually move the IMRC flaps and observe if the sensor’s voltage output changes smoothly and logically between its minimum and maximum range (e.g., 0.5V to 4.5V). Alternatively, use a bidirectional scan tool to monitor live data from the IMRC position sensor while commanding the actuator.
4. Actuator Function Test (Bidirectional Scan Tool): If available, use a professional scan tool with bidirectional control capabilities to command the IMRC actuator to cycle between its open and closed positions. Observe or listen for the actuator’s movement. If the actuator doesn’t respond or makes abnormal noises, it likely indicates a fault. While commanding the actuator, monitor the IMRC position sensor feedback (if equipped) to verify actual movement.
5. Vacuum System Test (for vacuum-actuated systems): Apply vacuum directly to the IMRC actuator using a hand-held vacuum pump. The actuator should hold vacuum and the runner flaps should move smoothly. Check the vacuum supply to the actuator under various engine conditions.

Recommended Repairs and Solutions

Based on the diagnostic findings, the following repairs are commonly indicated for P1520:

• Repair or Replace Damaged Wiring/Connectors: If visual inspection and electrical tests reveal damaged, corroded, or loose wiring or connectors, repair them using appropriate soldering techniques and heat-shrink tubing, or replace the entire sub-harness if damage is extensive.
• Replace Faulty IMRC Actuator: If electrical tests confirm an open or short circuit within the actuator, or if the actuator fails to respond to commanded inputs, replacement is necessary. Ensure the replacement part is compatible with the vehicle’s make and model.
• Replace Faulty IMRC Position Sensor: If the position sensor (if separate) provides inaccurate or no feedback despite the actuator functioning correctly, replace the sensor. In many applications, the sensor is integrated with the actuator.
• Clean or Replace Intake Manifold: If carbon buildup is causing the IMRC flaps to stick or bind, the intake manifold will need to be removed and thoroughly cleaned. In severe cases, or if the flaps themselves are damaged, the entire intake manifold assembly may require replacement. This is often a labor-intensive repair.
• Repair Vacuum Leaks: For vacuum-actuated systems, replace any cracked or damaged vacuum lines, check valves, or the IMRC vacuum diaphragm to restore proper vacuum supply and actuator function.
• PCM Replacement/Reprogramming: Only consider PCM replacement as a last resort, after all other components and wiring have been thoroughly tested and verified as functional. A qualified technician may also need to check for available PCM software updates that address known IMRC control issues.

After any repair, clear the DTCs from the ECM and perform a comprehensive test drive under varying engine conditions to ensure the issue is resolved and the IMRC system operates correctly without the P1520 code returning. Always use OEM or high-quality aftermarket parts for optimal performance and longevity.