What is OBD2 Code P1477 on a Chevrolet?
OBD2 diagnostic trouble code (DTC) P1477 is a manufacturer-specific code for General Motors (GM) vehicles, including Chevrolet, GMC, Cadillac, and Buick. It is defined as “EVAP Leak Detection Pump Control Circuit”. This code indicates a malfunction within the electrical control circuit of the vehicle’s Evaporative Emission Control (EVAP) system leak detection pump (LDP). The powertrain control module (PCM) has detected a voltage or signal issue—such as an open, short to ground, or short to power—in the circuit that controls the LDP’s operation. Unlike generic EVAP leak codes (like P0442), P1477 points specifically to an electrical fault in the pump’s control, not necessarily a physical leak in the fuel vapor system.
Technical Causes and Symptoms of P1477
When the PCM commands the Leak Detection Pump to activate during its self-test cycle, it monitors the circuit’s response. A P1477 code sets when the actual voltage or current in the circuit does not match the expected parameters. This prevents the EVAP system from properly pressurizing to check for leaks, potentially causing the vehicle to fail an emissions inspection.
Primary Causes of Chevrolet P1477 Code
- Faulty Leak Detection Pump (LDP): The most common cause. Internal motor failure, seized pump, or electrical failure within the pump assembly.
- Damaged Wiring or Connectors: Corroded, frayed, or broken wires in the LDP control circuit, often near the pump (located under the vehicle near the fuel tank).
- Blown Fuse: A dedicated fuse for the EVAP system or LDP circuit may be blown.
- Faulty Leak Detection Pump Relay: On models with a separate relay (often in the underhood fuse block), a stuck open or shorted relay can cause this code.
- Poor Electrical Ground: A corroded or loose ground connection for the LDP or PCM.
- Defective Powertrain Control Module (PCM): Rare, but a failed PCM driver for the LDP circuit is possible.
Common Symptoms of a P1477 Fault
- Illuminated Check Engine Light (MIL).
- Possible failed state emissions test.
- You may notice no other drivability symptoms; the vehicle often runs normally.
- In some cases, you might hear a continuous or intermittent humming noise from the rear of the vehicle if the pump is stuck on.
- Other EVAP-related codes (like P0442, P0455) may be present.
Step-by-Step Diagnostic Procedure for P1477
Proper diagnosis requires a digital multimeter (DMM) and a quality scan tool capable of performing bi-directional controls. Always begin by checking for technical service bulletins (TSBs) for your specific Chevrolet model year.
Step 1: Preliminary Inspection & Visual Check
Visually inspect the Leak Detection Pump, typically mounted on the frame rail near the fuel tank. Check its electrical connector for corrosion, bent pins, or moisture. Follow the wiring harness back, looking for chafing, cuts, or damage. Locate and check the relevant EVAP system fuse (consult your owner’s manual). Inspect the LDP relay if applicable.
Step 2: Circuit Testing with a Multimeter
Disconnect the electrical connector from the LDP. With the key in the ON position (engine off), check for power (12V) at the appropriate pin on the harness side. Check for continuity to ground on the ground circuit. Using a wiring diagram is crucial here. Next, test the LDP itself. Using the DMM in resistance (ohms) mode, measure the resistance across the pump’s motor terminals. A reading of infinite resistance (open) or zero resistance (short) indicates a failed pump. A typical LDP will show a specific resistance (e.g., 10-30 ohms).
Step 3: Active Command and Signal Testing
Using a bi-directional scan tool, command the Leak Detection Pump ON and OFF. You should hear and feel a distinct click from the pump relay (if equipped) and the pump may vibrate or hum briefly. If it does not activate, use your multimeter to check for voltage at the LDP connector while commanding it on. If voltage is present but the pump doesn’t run, the pump is faulty. If no voltage is present, the fault is in the wiring, relay, or PCM.
Step 4: Checking for Related Codes and Data
Clear the P1477 code and perform a complete drive cycle to see if it returns immediately or only after specific conditions. Monitor EVAP system status data PIDs with your scan tool to see if the PCM is attempting to run the leak test.
Repair Solutions and Cost Estimation
Once the faulty component is identified, repair is generally straightforward. Always disconnect the battery before performing electrical repairs.
Replacing the Leak Detection Pump
This is the most common fix. The LDP is usually held by one or two bolts and a quick-connect vapor hose. After disconnecting the electrical connector and hose, unbolt the old pump. Installation is the reverse. Important: Use a new gasket or O-ring if provided. After replacement, clear codes and perform an EVAP system monitor drive cycle.
Repairing Wiring and Connectors
For damaged wires, solder and heat-shrink the repair is the professional standard. Never use twist-on connectors in automotive environments due to vibration. For corroded connectors, consider a pigtail replacement harness.
Estimated Repair Cost for P1477
- DIY (Part Only): A new Leak Detection Pump typically costs between $80 and $200 depending on the Chevrolet model.
- Professional Repair: At a shop, total cost including 1 hour of labor can range from $200 to $400.
- Wiring repair costs are variable but generally less if no major harness replacement is needed.
Preventative Maintenance and Final Notes
While electrical failures are hard to predict, keeping the undercarriage clean and avoiding impact damage to components near the fuel tank can help. If you live in a rust-prone area, periodically inspecting electrical connectors for corrosion is wise. Remember, a P1477 code will prevent the EVAP monitor from running, which is a requirement for passing an OBD2-based emissions test in most regions. Addressing this code promptly ensures your Chevrolet remains compliant and environmentally sound.