P1477

What Does Code P1477 Mean?

DTC P1477 signifies a malfunction within the control circuit of the secondary or additional cooling fan relay. This code is typically manufacturer-specific, but generally indicates that the Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected an unexpected electrical state in the circuit responsible for activating the cooling system’s secondary fan, often designated for high-speed operation or supplemental cooling during air conditioning (A/C) use. The ECM/PCM actively monitors the voltage and current feedback from this circuit. When the module commands the secondary fan relay ON, it expects to see a specific voltage drop or current flow indicating the relay has closed and power is being supplied to the fan motor. Conversely, when the relay is commanded OFF, the ECM expects the circuit to return to a quiescent state. If the actual feedback deviates from the expected parameters – for instance, detecting a continuous open circuit, a short to ground, a short to battery voltage, or an incorrect resistance – it registers a fault and sets code P1477. This directly impacts the engine’s cooling subsystem, potentially leading to inadequate heat dissipation.

Common Symptoms

• Illuminated Malfunction Indicator Lamp (MIL): The Check Engine Light will be on.
• Engine Overheating: Particularly noticeable at idle, in heavy traffic, or at low vehicle speeds when natural airflow is insufficient and the secondary fan fails to engage.
• Reduced Air Conditioning Performance: The A/C system may not cool effectively, especially at idle, as the condenser fan (often the secondary cooling fan) is not assisting in heat rejection.
• Cooling Fan Inoperation: The secondary cooling fan may not turn on when commanded by the ECM/PCM, or it may run continuously.

What Causes the Code P1477?

• Faulty Secondary Cooling Fan Relay: The relay itself may have internal failures such as sticking contacts, an open coil, or a short circuit.
• Damaged or Corroded Wiring: An open circuit, short to ground, or short to voltage within the wiring harness connecting the ECM/PCM to the fan relay, or from the relay to the fan motor. This includes chafed insulation, corroded conductors, or damaged terminals.
• Corroded or Loose Electrical Connectors: Poor electrical contact at the fan relay socket, the fan motor connector, or any inline connectors in the cooling fan circuit.
• Blown Fuse: A fuse protecting the secondary cooling fan relay control circuit or the fan motor power circuit may be open.
• Faulty Cooling Fan Motor: While less common for a circuit fault code, an excessively high current draw due to internal motor wear or a complete open circuit within the motor windings can sometimes trigger relay circuit faults if the ECM is monitoring current.
• Internal ECM/PCM Malfunction: A rare possibility where the driver circuit within the ECM/PCM responsible for activating the relay has failed.

How to Diagnose and Troubleshoot

Diagnosis of P1477 requires a methodical approach, utilizing an OBD-II scanner, a digital multimeter (DMM), and vehicle-specific wiring diagrams.

1. Initial Scan and Freeze Frame Data Analysis: Connect an OBD-II scanner to confirm P1477. Review freeze frame data to understand engine temperature, vehicle speed, and A/C status at the time of fault. This provides crucial context for when the fault occurred. Clear the DTC and attempt to replicate the conditions.
2. Visual Inspection: Begin with a thorough visual inspection of the secondary cooling fan assembly, its wiring harness, and all associated connectors. Look for signs of physical damage, corrosion, melted insulation, loose terminals, or obstructions to the fan blades. Verify that all connectors are fully seated and locked. Locate and inspect the fan relay and its socket for corrosion or damage.
3. Fuse Verification: Identify all fuses associated with the cooling fan system (typically in the underhood fuse box). Using a DMM, check for continuity across each fuse or voltage drop across test points. Replace any blown fuses with one of the correct amperage. Note: A blown fuse often indicates an underlying short circuit.
4. Cooling Fan Relay Testing:
   • Relay Swap Test: If an identical, known-good relay from a non-critical circuit (e.g., horn relay, if compatible) is available, swap it with the secondary fan relay to see if the fault clears.
   • Bench Testing: Remove the relay. Using a 12V power source and a DMM, apply 12V to the relay coil terminals. Verify an audible click and check for continuity across the load terminals. Measure the resistance of the relay coil; compare to specifications. An open or extremely high resistance indicates an internal coil fault.
5. Circuit Voltage and Continuity Checks (with DMM):
   • Relay Socket Power (Load Side): With the ignition ON and the relay removed, use the DMM to check for battery voltage (B+) at the appropriate terminal in the relay socket that feeds the fan motor. This is typically a constant fused 12V supply.
   • Relay Socket Ground (Load Side): Check for continuity to chassis ground at the other load terminal in the relay socket (the wire going to the fan motor). This circuit is completed when the relay closes.
   • Relay Coil Power/Ground (Control Side): Identify the ECM/PCM control wire for the relay coil. Depending on the vehicle, the ECM/PCM may provide a switched ground or a switched 12V signal to activate the relay. Verify the presence of constant power or ground to the other coil terminal when the ECM/PCM is not commanding the fan on.
   • ECM/PCM Control Signal Test: With the relay removed, connect the DMM to the ECM/PCM control wire at the relay socket and a known good ground (or B+). Use the OBD-II scanner’s bi-directional controls to command the secondary fan ON/OFF. Observe the DMM for the expected voltage change (e.g., 0V to 12V, or 12V to 0V for ground-side switching).
   • Fan Motor Integrity: Disconnect the electrical connector at the secondary fan motor. Using the DMM, measure the resistance across the fan motor terminals. A low, measurable resistance (typically less than 2 ohms, consult specs) is expected. An open circuit (OL) indicates an internal motor winding fault. Check for continuity between each motor terminal and chassis ground; any continuity indicates a short within the motor.
   • Wiring Harness Integrity: Perform continuity checks on each wire segment between the ECM/PCM connector, relay socket, and fan motor connector. Simultaneously, check each wire for shorts to chassis ground and shorts to battery voltage.
6. Bi-directional Control Test (Advanced Scanner): If available, use the scanner’s bi-directional controls to directly command the secondary cooling fan ON and OFF. Monitor the fan’s operation and observe live data parameters related to fan status to confirm if the ECM/PCM is successfully sending the command and receiving feedback.

Recommended Repairs and Solutions

Once the root cause of P1477 has been precisely identified through systematic diagnosis, the appropriate repair can be performed.

• Replace Faulty Secondary Cooling Fan Relay: This is frequently the culprit. Ensure the replacement relay is the exact specified part number for your vehicle to guarantee correct function and current handling capacity.
• Repair or Replace Damaged Wiring Harness: Address any identified open circuits, shorts, or damaged insulation in the wiring. Use high-quality, vehicle-grade repair wire, proper soldering techniques (never crimp for long-term critical circuits), and heat-shrink tubing for durability. If the damage is extensive or in a critical junction, replacing the affected section of the wiring harness may be more reliable.
• Clean and Secure Electrical Connectors: Use specialized electrical contact cleaner to remove corrosion from terminals. For loose terminals, a terminal repair kit can be used to re-tension them or replace individual terminals. Ensure all connectors are fully seated and their locking tabs engaged.
• Replace Blown Fuse and Investigate Cause: If a blown fuse was discovered, replace it with a fuse of the correct amperage rating. If the fuse blows again immediately, a persistent short circuit in the fan motor or wiring is indicated and requires further diagnosis.
• Replace Faulty Cooling Fan Motor Assembly: If the fan motor itself tests open, shorted, or exhibits excessive resistance, the entire fan motor assembly usually needs to be replaced. Ensure the replacement part is compatible and correctly installed.
• ECM/PCM Replacement (Last Resort): Only consider ECM/PCM replacement if all other components and wiring have been exhaustively tested and verified to be in perfect working order. ECM/PCM replacement typically requires programming to the vehicle.

Mechanic’s Tips:
Always disconnect the battery before working on electrical components. When performing continuity or resistance checks, ensure the circuits are de-energized. Refer to manufacturer-specific service manuals for exact wiring diagrams, component locations, and resistance specifications, as these can vary significantly between models. After any repair, clear the DTCs, perform a functional test of the cooling fan system (e.g., let the engine idle until it reaches operating temperature, or run the A/C), and conduct a road test to confirm the fault does not return and the cooling system operates as intended.