P0079

What Does Code P0079 Mean?

DTC P0079 signifies “Exhaust Valve Control Solenoid Circuit Low (Bank 1)”. This code is set by the Engine Control Module (ECM), often referred to as the Powertrain Control Module (PCM), when it detects a voltage condition in the electrical circuit of the exhaust camshaft position actuator/control solenoid on Bank 1 that is lower than a pre-calibrated threshold. Bank 1 universally refers to the cylinder bank that contains cylinder #1. The exhaust valve control solenoid is an integral component of the Variable Valve Timing (VVT) or Variable Valve Lift (VVL) system. Its primary function is to regulate the flow of engine oil pressure to the exhaust camshaft phaser or actuator, thereby advancing or retarding the exhaust valve timing relative to the crankshaft position. The ECM continuously monitors the electrical characteristics of this solenoid’s circuit, including voltage, current, and resistance. When the ECM detects an abnormally low voltage reading, often indicative of an open circuit, a high-resistance connection, or a short to ground within the solenoid itself or its wiring, it interprets this as a fault in the circuit’s integrity and stores P0079, illuminating the Malfunction Indicator Lamp (MIL).

Common Symptoms

• Illuminated Malfunction Indicator Lamp (MIL): The Check Engine Light will be on.
• Reduced Engine Performance: Noticeable decrease in horsepower and torque, particularly during acceleration.
• Rough Engine Idle: The engine may run erratically or vibrate excessively at idle speeds.
• Increased Fuel Consumption: Inefficient valve timing can lead to poorer fuel economy.
• Difficulty Starting: Although less common, severely incorrect valve timing can sometimes affect engine cranking and starting.
• Engine Noise: In some cases, a rattling or ticking noise from the engine compartment may be heard if the VVT system is mechanically affected due to the electrical fault or a related underlying condition.

What Causes the Code P0079?

• Faulty Exhaust Valve Control Solenoid: The most common cause is an internal electrical failure within the solenoid, such as an open circuit in the winding, a short to ground, or excessive internal resistance.
• Wiring Harness Issues: Damage to the electrical wiring connected to the solenoid, including frayed wires, breaks, shorts to ground, or short circuits to other wires. Corrosion or degradation of the wire insulation can also lead to these issues.
• Corroded or Loose Electrical Connectors: Poor electrical connection at either the solenoid’s connector or the ECM’s corresponding connector pins due to corrosion, bent pins, or insufficient tension.
• ECM/PCM Failure: While rare, an internal fault within the ECM that affects its ability to properly supply voltage or interpret feedback from the solenoid circuit can cause this code. This is typically diagnosed after ruling out all other possibilities.
• Contaminated Engine Oil: Although not a direct electrical cause for a “circuit low” code, heavily contaminated engine oil can sometimes cause solenoids to stick or bind, leading to increased current draw or resistance that the ECM might interpret as a circuit fault over time, or even lead to internal solenoid failure. However, for P0079 specifically, the issue is electrical.

How to Diagnose and Troubleshoot

A systematic approach is crucial for accurately diagnosing P0079:

1. Visual Inspection:
   • Begin with a thorough visual inspection of the exhaust valve control solenoid, its wiring harness, and connectors for Bank 1. Look for obvious signs of damage, such as frayed or burnt wires, loose or corroded connectors, and oil leaks around the solenoid itself.
   • Inspect the engine oil level and condition. While not a direct cause of “circuit low,” proper oil pressure and cleanliness are vital for VVT operation.
2. OBD-II Scanner Diagnostics:
   • Connect an OBD-II scanner and retrieve all stored DTCs and freeze frame data. Freeze frame data provides the operating conditions (engine speed, load, temperature, etc.) at the moment the code was set, which can be invaluable.
   • Clear the code and perform a short test drive to see if it returns immediately.
   • Monitor live data parameters related to the exhaust valve control solenoid on Bank 1. Look for commanded duty cycle, actual cam position (if available), and any feedback voltage values. If the ECM commands the solenoid but there’s no corresponding change in voltage or cam position, it reinforces an electrical or mechanical fault.
   • Check for any other VVT-related codes (e.g., P0011, P0012, P0021, P0022) which might indicate a broader VVT system issue.
3. Digital Multimeter (DMM) Testing:
   • Solenoid Resistance Check: Disconnect the electrical connector from the exhaust valve control solenoid. Using a DMM set to ohms, measure the resistance across the solenoid’s terminals. Compare this reading to manufacturer specifications (typically between 6-12 ohms, but consult service data for the specific vehicle). An open circuit (infinite resistance) or a reading significantly outside the specified range indicates a faulty solenoid.
   • Power and Ground Supply Check: With the ignition in the “ON” position (engine off) and the solenoid connector disconnected, use the DMM to check for voltage at the power supply pin of the harness connector (usually battery voltage) and for proper ground at the ground pin. Ensure a clean, strong ground connection.
   • Continuity and Short to Ground Check (Wiring Harness): Disconnect the battery and both ends of the wiring harness (at the solenoid and at the ECM). Use the DMM in continuity mode to check for continuity between the solenoid connector pins and their corresponding ECM connector pins. Check for shorts to ground by placing one DMM lead on the harness wire terminal and the other on a known good chassis ground. Check for shorts to power by checking continuity between the harness wire and any battery positive wire.
4. Solenoid Actuation Test (if applicable):
   • If your scanner has bi-directional control capabilities, attempt to actuate the exhaust valve control solenoid on Bank 1. Listen for an audible click from the solenoid. A lack of clicking could indicate an electrical fault within the solenoid or a power/ground issue to it.

Recommended Repairs and Solutions

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

• Replace Exhaust Valve Control Solenoid: If the DMM tests confirm an internal fault with the solenoid (open circuit, short, or incorrect resistance), replacement of the exhaust valve control solenoid for Bank 1 is necessary. Always use an OEM or high-quality aftermarket part, as cheaper alternatives can fail prematurely.
• Repair or Replace Wiring Harness: If the diagnostic steps identify damaged, corroded, or shorted wiring, the affected sections of the harness should be repaired or replaced. This may involve soldering new wires, repairing connectors, or, in severe cases, replacing a segment of the engine wiring harness. Ensure all repairs are properly insulated and sealed to prevent future issues.
• Clean and Secure Electrical Connectors: If corrosion or loose pins are found at the solenoid or ECM connector, clean the pins thoroughly using electrical contact cleaner and a small brush. Ensure secure pin tension and apply dielectric grease to protect against future corrosion before reassembling.
• ECM/PCM Replacement: This is a last resort and should only be considered if all other components (solenoid, wiring, connectors) have been thoroughly tested and confirmed to be in perfect working order, and specific ECM output tests indicate an internal fault. ECM replacement often requires specialized programming or flashing to the vehicle’s VIN and options.
• Post-Repair Verification: After any repair, clear the DTCs from the ECM. Perform an extended drive cycle under various operating conditions to ensure the P0079 code does not return and that the VVT system is operating correctly.