Code P0079: Low Circuit Voltage for Bank 1 Exhaust Valve Control Solenoid
Definition and Meaning of Fault Code P0079
The generic code P0079 is an OBD-II code related to the powertrain. It indicates that the Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected an abnormally low voltage in the electrical circuit of the exhaust valve control solenoid on engine Bank 1.
On vehicles equipped with a Variable Valve Timing (VVT) system, the ECM/PCM uses this solenoid, controlled by a PWM (Pulse Width Modulation) signal, to regulate oil pressure and thus adjust the camshaft position. When the measured voltage in this circuit is below the manufacturer’s specifications, code P0079 is stored and the Malfunction Indicator Lamp (MIL) illuminates.
Important Note: “Bank 1” (B1) refers to the side of the engine where cylinder number 1 is located. Always consult your vehicle’s technical documentation to locate it precisely. This code is often associated with other faults such as P0027.
Common Symptoms of Fault P0079
When code P0079 is active, you may observe one or more of the following symptoms:
- Malfunction Indicator Lamp (MIL) illuminated on the dashboard.
- Noticeable reduction in vehicle performance and acceleration.
- Increased fuel consumption.
- Irregular idle or rough idle.
- In some cases, the vehicle may enter limp mode to protect the engine.
Potential Causes of Code P0079
Several issues can cause low voltage in the solenoid circuit:
- A short circuit to ground in the solenoid wiring harness.
- The VVT control solenoid itself is faulty and internally shorted.
- A faulty, corroded, or loose electrical connector.
- A blown fuse or an electrical supply problem.
- In rare cases, a faulty ECM/PCM module.
Diagnostic Procedure for Code P0079
A methodical approach is essential to identify the root cause. Always consult the specific electrical diagrams for your vehicle before any intervention.
1. Visual Inspection and Connector Check
Start with a thorough visual inspection. Disconnect the connectors at the solenoid and the ECM/PCM. Look for signs of corrosion, pin damage, loose or melted wires.
2. Wiring Harness Check
Using a digital multimeter (DVOM):
- Disconnect the solenoid connector and the ECM connector.
- Check the continuity and resistance of the wires between the two connectors. Abnormally high resistance indicates a poor connection.
- Test each wire in the circuit to check for the absence of a short to ground (place one multimeter lead on a good ground and the other on the wire; the resistance should be infinite).
3. VVT Control Solenoid Test
Test the internal resistance of the solenoid according to the manufacturer’s reference values. Place one multimeter lead on a good ground and the other on each terminal of the solenoid. Very low or zero resistance indicates a shorted and faulty solenoid.
4. Control Signal and ECM Check
If the wiring and solenoid are intact:
- Reconnect all connectors.
- Using an advanced scan tool, activate the solenoid and observe its commanded duty cycle.
- Using an oscilloscope or a multimeter measuring duty cycle, probe the circuit at the solenoid to verify that the signal sent by the ECM matches the command. A missing or inconsistent signal may indicate an ECM/PCM failure.
Repair and Prevention
Once the cause is identified, proceed with the repair: replace the solenoid, repair the wiring harness, or in rare cases, replace the ECM. After repair, clear the fault codes and perform a road test to ensure code P0079 does not reappear.
Pro Tip: Use engine oil with the viscosity and specification recommended by the manufacturer. Dirty or unsuitable oil can disrupt the VVT system operation and cause faults.
Need Help?
Electrical diagnosis can be complex. If you do not have the necessary equipment or expertise, consult a professional mechanic for an accurate diagnosis and reliable repair. Avoid driving with this active fault to prevent potential engine damage.