Understanding the Buick P1479 Diagnostic Trouble Code
When your Buick’s check engine light illuminates and a scan tool reveals code P1479, you’re dealing with a specific fault within the vehicle’s Evaporative Emission Control (EVAP) system. This code is formally defined as “EVAP Leak Detection Pump Control Circuit”. Unlike generic EVAP leak codes (like P0442), P1479 points directly to an electrical or control issue with the component responsible for actively testing the system for leaks: the Leak Detection Pump (LDP). This code is common on many GM vehicles, including popular Buick models like the Century, Regal, and LeSabre from the late 1990s through the 2000s. A proper diagnosis requires a solid understanding of the EVAP system’s function and its electrical control network.
What is the EVAP Leak Detection Pump (LDP)?
The Leak Detection Pump is a critical, electronically-controlled component within the EVAP system. Its primary job is not to manage fuel vapors during normal driving, but to perform a self-test. When conditions are right (typically with a cool engine, between 1/4 and 3/4 fuel level, and after a specific drive cycle), the Powertrain Control Module (PCM) activates the LDP. The pump pressurizes the sealed fuel vapor system with air and then monitors the pressure decay to determine if a leak larger than the legal limit (usually 0.020″ or 0.5mm) is present. Code P1479 indicates the PCM has detected an unexpected voltage condition in the electrical circuit that controls this pump.
How the PCM Monitors the LDP Control Circuit
The PCM uses a sophisticated feedback loop to control the LDP. It sends a Pulse Width Modulated (PWM) signal to the pump’s solenoid to turn it on and off. Simultaneously, it monitors the voltage in this control circuit. If the PCM commands the LDP on but sees a voltage that is too high (indicating an open circuit or poor connection), or if it commands the LDP off but sees a voltage that is too low (indicating a short to ground), it will set code P1479. This is a “circuit-level” fault, meaning the problem lies in the wiring, connectors, or the pump’s internal solenoid coil, not necessarily in the mechanical function of the pump itself.
Symptoms and Common Causes of P1479 in Buick Models
A Buick with a stored P1479 code may not exhibit dramatic driving performance issues, as the EVAP system is primarily for emissions control. However, the symptoms and underlying causes are important for diagnosis and to prevent an emissions test failure.
Primary Symptoms of Code P1479
- Illuminated Check Engine Light (MIL): This is the most common and often the only noticeable symptom.
- Failed Emissions Inspection: The vehicle will not pass a state-mandated OBD2 emissions test with an active P1479 code.
- Possible EVAP System Test Failure: The PCM will be unable to complete its mandatory self-test for the EVAP system, potentially leading to other codes.
- No Fuel Smell or Performance Loss: Unlike a large EVAP leak, this electrical fault typically does not cause a gasoline odor or affect engine running.
Most Frequent Causes of P1479
- Faulty Leak Detection Pump (LDP): The internal solenoid winding can open or short, causing a direct circuit failure.
- Damaged Wiring or Connectors: Corrosion, chafing, or rodent damage to the wiring harness between the PCM and the LDP.
- Poor Electrical Connections: A corroded or loose connector at the LDP or at the PCM can create high resistance or an intermittent open circuit.
- Blown Fuse: The LDP circuit is often protected by a fuse in the underhood fuse block. A blown fuse will create an open circuit.
- Rare PCM Failure: While less common, a fault within the PCM’s driver circuit for the LDP control can be the culprit.
Step-by-Step Diagnostic Procedure for P1479
Diagnosing P1479 requires a methodical approach, starting with the simplest checks and progressing to more complex electrical tests. You will need a basic digital multimeter (DMM) and possibly a scan tool capable of activating the LDP for a functional test.
Step 1: Preliminary Visual and Basic Checks
Begin by locating the Leak Detection Pump. On most front-wheel-drive Buicks (like the Century/Regal), it is often mounted near the fuel tank or behind a rear wheel well liner. On others, it may be in the engine bay.
- Visually inspect the LDP, its wiring harness, and connectors for obvious damage, corrosion, or disconnection.
- Check the relevant fuse (consult your owner’s manual or service information for the exact fuse location, often labeled “EVAP” or “ECM”).
- Clear the code and perform a complete drive cycle to see if it returns immediately (indicating a hard fault) or intermittently.
Step 2: Electrical Circuit Testing with a Multimeter
With the vehicle’s ignition OFF and the LDP connector disconnected, use your multimeter to perform these key tests:
- Check for Power (B+): Probe the LDP harness connector’s power wire (usually a 12V feed, often pink/black). With the ignition in RUN (engine off), you should see battery voltage.
- Check for Ground: Probe the ground wire (usually black) and verify a good connection to chassis ground (less than 0.5 ohms resistance).
- Check the Control Circuit: Measure the resistance of the LDP’s internal solenoid by probing the two terminals on the pump itself. A typical reading should be between 10 and 30 ohms. A reading of infinite resistance (OL) indicates an open coil. A reading near 0 ohms indicates a shorted coil.
Step 3: Active Control Signal and Functional Test
This is the most definitive test. Reconnect the LDP connector. Using a scan tool with bidirectional controls, access the PCM output controls and command the LDP “ON.” You should feel and hear a distinct clicking from the pump. If you don’t, back-probe the control wire with your multimeter set to DC Volts. When commanded ON, you should see the voltage change (often dropping from a reference voltage to near 0V). No change indicates a PCM or wiring issue; a change with no pump action confirms a faulty LDP.
Repair Solutions and Final Verification
Once the root cause is identified, the repair is typically straightforward. Always disconnect the negative battery cable before performing electrical repairs.
Common Repair Procedures
- Replacing the Leak Detection Pump: If the pump’s solenoid tests bad, replacement is the only option. Ensure you get the correct part for your specific Buick model and year.
- Repairing Wiring: For damaged wires, solder and heat-shrink the repair. Never use twist-on connectors in automotive environments.
- Cleaning Connectors: Use electrical contact cleaner and a small brush to remove corrosion from terminals. Apply dielectric grease to prevent future corrosion.
- Replacing a Fuse: If a fuse is blown, replace it with one of the identical amperage rating. If it blows again, you have a short to ground in the circuit that must be found.
Post-Repair Steps and Clearing the Code
After completing the repair, reconnect the battery. Start the engine and use your scan tool to clear the P1479 diagnostic trouble code from the PCM’s memory. The check engine light should turn off. To ensure a complete fix, the vehicle must undergo a full EVAP monitor drive cycle. This involves a specific pattern of driving (cold start, mixed city/highway driving) to allow the PCM to run the leak detection test successfully. Use your scan tool to monitor the “EVAP Monitor” status; when it shows “Ready” or “Complete,” you have verified the repair is successful and the system is fully functional.