Understanding the GMC P1479 Diagnostic Trouble Code
The OBD2 trouble code P1479 is a manufacturer-specific code primarily associated with General Motors (GM) vehicles, including GMC trucks and SUVs. Its official definition is “EVAP Leak Detection Pump Heater Circuit” or sometimes “EVAP Leak Detection Pump Circuit.” This code is directly related to your vehicle’s Evaporative Emission Control (EVAP) system, a critical component for reducing harmful fuel vapor emissions and ensuring optimal engine performance. When the Powertrain Control Module (PCM) detects an electrical fault within the circuit of the Leak Detection Pump (LDP) or its integrated heater, it will log code P1479 and illuminate the Check Engine Light.
The EVAP system’s primary job is to capture fuel vapors from the fuel tank and prevent them from escaping into the atmosphere. The Leak Detection Pump is a smart component used in many GM applications to both create a vacuum for leak testing and to monitor the system’s integrity. A failure in its circuit prevents the PCM from performing this crucial self-test, potentially allowing undetected leaks and causing the vehicle to fail an emissions inspection.
Primary Causes of the P1479 Code in GMC Vehicles
Diagnosing a P1479 code requires a systematic approach, as the root cause can range from a simple blown fuse to a failed pump. The issue is almost always electrical in nature, related to the power, ground, or control signal of the Leak Detection Pump assembly.
1. Faulty Leak Detection Pump (LDP) Assembly
This is the most common culprit. The LDP itself is an electro-mechanical device containing a pump motor, solenoid valves, and often an internal heater element (for cold climate operation). Over time, the internal motor can fail, the solenoid can become stuck, or the heater circuit can short or open. Physical damage or corrosion can also lead to a complete pump failure.
2. Electrical Circuit Problems
Problems in the wiring harness are frequent causes. These include:
- Open or Shorted Wires: Damaged, corroded, or broken wires in the LDP circuit.
- Poor Electrical Connections: Corrosion or looseness at the LDP connector or PCM connector.
- Blown Fuse: The LDP is powered through a dedicated fuse in the underhood fuse block. A blown fuse will immediately disable the pump.
3. Failed or Corroded LDP Connector
The electrical connector that plugs into the Leak Detection Pump is exposed to road debris, moisture, and salt. Pins can become bent, corroded, or the plastic housing can crack, leading to an intermittent or complete loss of electrical connection.
4. Issues with the Powertrain Control Module (PCM)
While less common, a failure within the PCM itself—specifically in the driver circuit that controls the LDP—can trigger a P1479 code. This is typically considered only after all other wiring and component checks have been verified.
5. Vacuum Line or Hose Issues
Although P1479 is an electrical circuit code, physical damage to the vacuum lines connected to the LDP (cracks, disconnections) can sometimes cause related symptoms and should be inspected during diagnosis.
Step-by-Step Diagnostic Procedure for P1479
Proper diagnosis requires a digital multimeter (DMM) and a reliable scan tool. Always begin by verifying the code and checking for technical service bulletins (TSBs) for your specific GMC model year.
Step 1: Preliminary Inspection
- Visually inspect the Leak Detection Pump (usually located near the fuel tank or in the engine bay rear).
- Check the associated fuse (refer to your owner’s manual for location and rating).
- Examine the LDP electrical connector and vacuum lines for obvious damage, corrosion, or disconnections.
Step 2: Electrical Circuit Testing
With the connector disconnected from the LDP and the ignition ON (engine OFF), use your multimeter to check for:
- Power Supply (B+): One pin should have battery voltage (typically 12V).
- Ground Circuit: Another pin should show continuity to a good chassis ground.
- Control Signal: Using a scan tool with bidirectional controls, command the LDP ON/OFF. You should see the voltage on the control pin change accordingly.
If power and ground are present but the control signal is missing, suspect a PCM or wiring issue. If power is missing, trace the circuit back to the fuse.
Step 3: Testing the Leak Detection Pump Itself
If the electrical circuits test good, the LDP itself is likely faulty. You can perform a bench test by applying battery voltage and ground directly to the pump’s terminals (consult a wiring diagram for your model). A functioning pump should audibly click or run. Warning: Do not apply voltage randomly; you risk damaging a good component.
Step 4: Scan Tool Data Monitoring
Use your scan tool to monitor EVAP test status and LDP command parameters. Look for discrepancies or a failure to complete the EVAP monitor due to the LDP circuit fault.
Repair Solutions and Cost Considerations
Once the faulty component is identified, the repair is typically straightforward.
Replacing the Leak Detection Pump
Replacement is the most common fix. The LDP is usually held by a bracket with a few bolts. The process involves:
- Disconnecting the electrical connector and vacuum lines.
- Unbolting the pump from its bracket.
- Installing the new unit and reconnecting everything.
After replacement, clear the codes with your scan tool and perform an EVAP monitor drive cycle to ensure the system passes its self-test.
Repairing Wiring Harness Damage
For damaged wires, solder and heat-shrink tubing provide the most reliable and permanent repair. Avoid using simple crimp connectors or electrical tape in the vehicle’s undercarriage due to exposure elements.
Estimated Repair Costs
Costs can vary widely:
- DIY Repair: A new Leak Detection Pump typically costs between **$80 and $200** for the part. Add the cost of a fuse or wiring repair materials.
- Professional Repair: At a shop, total costs (parts and labor) often range from **$200 to $400**, depending on labor rates and the vehicle’s accessibility.
Ignoring a P1479 code will not cause immediate drivability issues, but it will prevent the EVAP system from checking for leaks, potentially leading to increased emissions, a failed state inspection, and a slight decrease in fuel efficiency over time.