What is OBD-II Code P1479 in a Mercury Vehicle?
OBD-II (On-Board Diagnostics II) code P1479 is a manufacturer-specific diagnostic trouble code (DTC) that is primarily associated with vehicles from the Ford Motor Company family, which includes Mercury and Lincoln brands. The generic definition is “Leak Detection Pump Circuit High.” This code indicates that the vehicle’s Powertrain Control Module (PCM) has detected an electrical fault in the circuit for the Leak Detection Pump (LDP), a critical component of the Evaporative Emission Control (EVAP) system. Specifically, the PCM is reading a voltage signal from the LDP circuit that is higher than the expected normal operating range.
The Role of the EVAP System and Leak Detection Pump
The EVAP system is designed to prevent fuel vapors from the gas tank from escaping into the atmosphere. It captures these vapors, stores them in a charcoal canister, and then purges them into the engine to be burned during normal combustion. The Leak Detection Pump is the system’s self-diagnostic tool. It pressurizes the EVAP system with air and then monitors for a pressure drop, which would indicate a leak. A fault in this pump or its circuit compromises the vehicle’s ability to self-test for emissions leaks.
Why Code P1479 is Specific to Ford/Mercury
While the P1XXX series of codes are manufacturer-specific, P1479 is almost exclusively found on Ford, Mercury, and Lincoln vehicles from the late 1990s through the 2000s. This is because the code relates to the particular design and electrical strategy Ford employed for its EVAP system monitoring, often using a dedicated Leak Detection Pump rather than the more common method of using the fuel tank pressure sensor.
Symptoms and Causes of a P1479 Code
When code P1479 is stored in the PCM, it will illuminate the Check Engine Light (MIL). In many cases, this may be the only noticeable symptom, as the EVAP system fault typically does not affect drivability or fuel economy in a direct, noticeable way. However, a persistent EVAP leak can lead to a faint fuel odor.
Common Symptoms of P1479
- Illuminated Check Engine Light (MIL): The primary and most common symptom.
- Failed Emissions Test: The vehicle will not pass a state emissions inspection with an active EVAP system code.
- Possible Fuel Odor: If the fault is related to an actual leak, you may smell gasoline vapors, especially near the fuel tank or filler neck.
- Other EVAP Codes: P1479 may appear alongside other codes like P0455 (Large EVAP Leak) or P0442 (Small EVAP Leak) if the LDP failure prevents proper leak testing.
Primary Causes of Code P1479
The “Circuit High” designation points to an electrical problem. The root causes generally fall into three categories:
- Electrical Faults: A short to power in the LDP control circuit, damaged or corroded wiring/connectors, or a poor electrical ground for the LDP or PCM.
- Failed Leak Detection Pump (LDP): An internal failure of the pump motor or its integrated solenoid valve. The pump is often located near the EVAP canister, behind a rear wheel well or under the vehicle.
- Faulty Powertrain Control Module (PCM): While rare, a malfunction within the PCM itself could cause it to misread the circuit voltage. This should only be considered after all other possibilities are eliminated.
Step-by-Step Diagnostic and Repair Procedure
Diagnosing P1479 requires a systematic approach, starting with the simplest checks and moving toward component replacement. You will need a quality OBD2 scan tool and a digital multimeter (DMM).
Step 1: Preliminary Inspection and Code Verification
Begin by performing a thorough visual inspection. Look for obvious issues in the EVAP system area:
- Check the LDP electrical connector for corrosion, bent pins, or damage.
- Inspect the wiring harness from the LDP back to the PCM for chafing, cuts, or rodent damage.
- Examine the EVAP hoses connected to the LDP and canister for cracks, disconnections, or brittleness.
- Clear the code with your scan tool and perform a drive cycle to see if it returns. An intermittent fault may point to a wiring issue.
Step 2: Electrical Circuit Testing
Consult a vehicle-specific wiring diagram to identify the LDP connector pins. Typically, the LDP will have a power supply wire (often from a relay or fuse), a ground wire, and a control wire from the PCM.
- With the key on, engine off, use the DMM to check for battery voltage at the LDP power supply pin.
- Check the ground circuit for continuity to a known good ground.
- Test the PCM control wire for continuity to the PCM connector and check for a short to battery voltage (which would cause the “Circuit High” condition).
Step 3: Testing the Leak Detection Pump Itself
If the wiring checks out, the LDP itself is likely faulty. You can perform a basic bench test:
- Disconnect the electrical and vacuum hoses from the LDP.
- Using fused jumper wires, apply direct battery voltage and ground to the pump’s motor terminals. A functioning pump should run audibly.
- Listen for the internal solenoid valve to click when voltage is applied to its circuit (if separately accessible).
- If the pump does not activate, it is confirmed dead and requires replacement.
Step 4: Replacement and System Verification
Replacing the LDP is generally straightforward. Ensure the vehicle is cool and safely supported if working underneath.
- Disconnect the battery negative terminal.
- Unplug the electrical connector and carefully label then disconnect the vacuum hoses.
- Remove the mounting bolts, swap the unit, and reassemble in reverse order, ensuring all hoses are securely connected.
- Reconnect the battery, clear all codes with your scan tool, and perform a complete drive cycle to allow the PCM to run its EVAP monitor. The monitor should complete without setting P1479 again.
Technical Considerations and Final Thoughts
Successfully repairing a P1479 code restores your Mercury’s emissions integrity and ensures it can pass mandatory inspections. It’s a repair that, while technical, is very achievable for a seasoned DIYer with the right tools.
Importance of Using OEM or High-Quality Parts
The Leak Detection Pump is a precision component. While aftermarket options exist, opting for an OEM (Motorcraft) or high-quality replacement part is highly recommended. Inferior pumps may have a shorter lifespan or not meet the exact flow/pressure specifications of the PCM, potentially leading to other EVAP codes or test failures.
When to Seek Professional Help
If your electrical diagnostics point toward a wiring fault deep in the harness or you suspect a PCM issue, the complexity increases significantly. A professional automotive technician with access to advanced diagnostic software, wiring schematics, and the ability to perform PCM programming or component tests may be required to resolve the issue efficiently and correctly.
In summary, OBD2 code P1479 is a clear call to action for your Mercury’s EVAP system health. By following a logical diagnostic path—inspecting wiring, testing the circuit, and verifying pump operation—you can pinpoint the fault, whether it’s a simple corroded connector or a failed Leak Detection Pump, and restore your vehicle to proper working order.