Technical Description of OBD-II Trouble Code
The circuit for controlling the exhaust valve solenoid is experiencing low voltage (Bank 1).
What does that mean?
This code is a generic OBD-II powertrain code that applies to all vehicle makes and models (1996-newer). However, the specific repair steps may vary depending on the model.
In vehicles with variable valve timing (VVT), the Engine Control Module / Powertrain Control Module (ECM/PCM) regulates the camshaft position by controlling the engine oil flow through the control solenoid. The control solenoid is activated by a Pulse Width Modulated signal (PWM) from the ECM/PCM. If the voltage of this signal falls below the specified range, the ECM/PCM will set this trouble code and activate the Malfunction Indicator Lamp (MIL).
Bank 1 refers to the side of the engine with cylinder #1. It is important to verify this according to the manufacturer’s specifications. The exhaust valve control solenoid is typically located on the exhaust manifold side of the cylinder head. This code is similar to codes P0078 and P0080 and may also be accompanied by P0027.
Symptoms
The symptoms may include:
Potential Causes
The potential causes of a P0079 diagnostic trouble code may include:
Diagnostic Steps
Wire harness – Disconnect the harness connectors from the PCM/ECM. Using a wiring diagram, locate the + and – wires connected to the solenoid. Depending on the application, the solenoid can be either ground side or power side controlled. Consult the factory wiring diagrams to determine the power flow in the circuit. Using a digital volt ohm meter (DVOM) set to the volts setting, check for voltage by placing the positive lead on the vehicle battery and the negative lead on each wire connected to the control solenoid. If the solenoid is grounded to the chassis, test the power lead to the control solenoid at the PCM/ECM wiring harness. There should be no voltage present. If voltage is detected, unplug connectors and trace the wiring back to the solenoid to check for a short to ground.
Control solenoid – Check for a short to ground through the control solenoid by placing one DVOM lead on a known good ground and the other lead on each terminal of the control solenoid. If there is a low resistance, the solenoid may be internally shorted.
PCM/ECM – If all wiring and the control solenoid check out okay, it will be necessary to monitor the solenoid during engine operation by back probing the wires at the PCM/ECM. Use an advanced scan tool that can read the engine functions to monitor the duty cycle commanded to the control solenoid. Connect an oscilloscope or graphing multimeter set to the duty cycle scale, with the negative lead connected to a known good ground and the positive lead connected to either wire terminal at the solenoid. The reading on the multimeter should match the commanded duty cycle on the scan tool. If they are opposite, the polarity may be reversed. Connect the positive lead to the other wire terminal on the solenoid and re-test to verify. If no signal is detected from the PCM, there may be a fault with the PCM itself.
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