Understanding the P1477 Diagnostic Trouble Code in Chrysler Vehicles
The OBD2 diagnostic trouble code P1477 is a manufacturer-specific code for Chrysler, Dodge, Jeep, and Ram vehicles. It is defined as “Leak Detection Pump Solenoid/Circuit”. This code is directly related to the vehicle’s Evaporative Emission Control (EVAP) system, which is designed to prevent fuel vapors from escaping into the atmosphere. The Leak Detection Pump (LDP) is a critical component that pressurizes the fuel system to check for leaks. When the Powertrain Control Module (PCM) detects an electrical fault within the LDP’s control circuit—such as a voltage reading that is too high (open) or too low (short)—it will log code P1477 and illuminate the Check Engine Light.
Role of the EVAP System and Leak Detection Pump
The EVAP system captures fuel vapors from the fuel tank and temporarily stores them in the charcoal canister. During specific driving conditions, the PCM purges these vapors into the engine to be burned. The Leak Detection Pump’s job is to perform a self-test. It uses engine vacuum to draw in air, then uses an internal solenoid to seal and pressurize the EVAP system. By monitoring the pressure, the PCM can determine if there is a leak. A fault in the LDP’s electrical circuit prevents this test from occurring, leading to the P1477 code.
Common Symptoms and Causes of Code P1477
Unlike some engine codes, P1477 may not always cause obvious drivability issues. Its primary symptom is the illumination of the Malfunction Indicator Lamp (MIL), or Check Engine Light. In some cases, you may notice a failed emissions test, as the EVAP monitor will be incomplete. Since the EVAP system is sealed, you typically will not smell fuel vapors for this specific circuit code.
Primary Causes of a P1477 Fault
- Faulty Leak Detection Pump (LDP): The internal solenoid can fail electrically (open or shorted windings) or mechanically.
- Damaged Wiring or Connectors: Corroded, broken, or shorted wires in the LDP circuit between the pump and the PCM.
- Poor Electrical Connections: A loose or corroded connector at the LDP or PCM can cause intermittent circuit faults.
- Blown Fuse: A fuse supplying power to the LDP or the PCM’s output driver circuit may be blown.
- Rare PCM Failure: Although less common, a faulty driver circuit inside the PCM itself can trigger this code.
Impact on Vehicle Operation and Emissions
The immediate impact of a P1477 code is the disabling of the EVAP leak detection monitor. This means the vehicle’s computer cannot verify the integrity of the fuel vapor system. While the car will still run normally, it will not pass an OBD2 emissions inspection. It is important to address this code to ensure the vehicle is operating as designed and to prevent the possibility of a larger, undetected fuel vapor leak.
Step-by-Step Diagnostic Procedure for P1477
Diagnosing P1477 requires a systematic approach, starting with the simplest checks and moving toward more complex tests. You will need a quality digital multimeter (DMM) and possibly a scan tool capable of performing actuator tests.
Step 1: Preliminary Visual Inspection
- Locate the Leak Detection Pump. It is usually found in the engine bay, near the charcoal canister or behind a fender liner.
- Visually inspect the LDP wiring harness for obvious damage, chafing, or burns.
- Check the electrical connector at the LDP for corrosion, bent pins, or looseness. Disconnect it and look for green/white corrosion.
- Consult your vehicle’s fuse diagram and check all relevant fuses (often labeled “PCM” or “ECM”).
Step 2: Electrical Circuit Testing
With the LDP connector disconnected and the ignition ON (engine OFF), use your multimeter to check for power and ground.
- Check for Power (Reference Voltage): One pin in the harness connector should show approximately 5 volts (this is a reference signal from the PCM).
- Check the Ground Circuit: Another pin should show continuity to a good chassis ground.
- Check the Signal Circuit: The third pin is the control circuit from the PCM. Testing this may require back-probing with the connector attached and using a scan tool to command the LDP on/off while monitoring voltage change.
Step 3: Testing the Leak Detection Pump Itself
If the wiring checks out, the next step is to test the LDP solenoid’s resistance.
- Set your multimeter to Ohms (Ω).
- Measure the resistance across the two main terminals of the LDP solenoid (consult a service manual for exact pinout).
- A typical reading should be between 10 and 30 ohms. A reading of 0 ohms indicates a short, and a reading of infinite resistance (OL) indicates an open circuit—both mean the LDP is faulty.
- You can also perform a bench test by applying 12 volts directly to the pump terminals (briefly); you should hear and feel a distinct click.
Repair Solutions and Prevention Tips
Once the root cause is identified, the repair is typically straightforward. The most common fix is replacing the Leak Detection Pump assembly.
Common Repair Procedures
- Replacing the LDP: This is the most frequent repair. Ensure you get an OEM or high-quality aftermarket part. Installation usually involves a few bolts and an electrical connector. Be sure to clear the code after replacement.
- Repairing Wiring: For damaged wires, solder and heat-shrink the repair. Never use twist-on connectors in automotive environments due to vibration.
- Cleaning Connectors: Use electrical contact cleaner and a small brush to remove corrosion from connector pins.
- Replacing a Fuse: If a fuse is blown, replace it. If it blows again, you have a short to ground in the circuit that must be found.
Clearing the Code and Verifying the Repair
After completing the repair, clear the DTCs with your scan tool. The Check Engine Light will turn off. To ensure the repair is successful, you need to complete an EVAP monitor drive cycle. This involves driving the vehicle under specific conditions (often a mix of city and highway driving) until the PCM runs its self-test. Use your scan tool to check the “Monitor Status” and confirm the EVAP monitor shows “Ready” or “Complete,” not “Incomplete” or “Failed.”
Preventive Maintenance Advice
While LDP failures are often unpredictable, you can minimize risks. Routinely inspect the under-hood area and undercarriage for damaged wiring, especially if you drive on rough roads. During routine maintenance, a quick visual check of the LDP and its connector can help catch early signs of corrosion or physical damage.