Technical Description of OBD-II Trouble Code
Malfunction in the Power/Ground Circuit of the Cooling Fan
What does that signify?
This generic diagnostic trouble code for powertrain/engines typically applies to all engines equipped with OBDII, but it is more common in certain Mazda and Mercedes Benz vehicles.
The Cooling Fan Module is usually located behind the radiator in front of the engine. The Powertrain Control Module (PCM) typically controls the Cooling Fan Module based on the wired inputs it receives.
The PCM receives voltage signals from various sources to determine engine coolant and intake air temperature, air conditioner pressures, and vehicle speed. This code is triggered if these inputs do not match the normal operating voltages stored in the PCM’s memory, or if the cooling fan operation does not match the predetermined information stored in the PCM.
P0485 is set when abnormal voltages are detected during normal fan operation on the power side/ground side of the cooling fan circuit. This can occur during low, medium, or high-speed operation. This also includes vehicles that use fan relays to operate the cooling fans. This code only deals with electrical issues (failed cooling fan circuit), including intermittent problems.
Troubleshooting steps may vary depending on the manufacturer, type of cooling fan module, and wire colors.
Related cooling fan trouble codes include:
Severity & Symptoms
The severity depends on the type of failure. An electrical failure may not be considered severe as the PCM can compensate for it, unless the engine starts to overheat. While the PCM is compensating, the temperature gauge must be monitored during all drive cycles. Typically, the fans operate continuously (100% duty cycle).
Symptoms of a P0485 trouble code may include:
Potential Causes
The typical causes for this code to be triggered are:
Diagnostic and Repair Procedures
A good starting point is always to search for a technical service bulletin (TSB) for your specific vehicle. The vehicle manufacturer may have a PCM flash/reprogram to address this issue, and it is worth checking before going down a potentially incorrect path.
Next, locate the cooling fan module on your specific vehicle. The Cooling Fan Module (cooling fans) is usually found mounted behind the radiator in front of the engine. Once located, visually inspect the connectors and wiring. Look for any scraping, rubbing, bare wires, burn spots, or melted plastic. Disconnect the connectors and carefully inspect the terminals inside. Check for any signs of burning or corrosion. If needed, clean the terminals using electrical contact cleaner and a plastic bristle brush. Allow them to dry and apply electrical grease to the contact points.
If you have a scan tool, clear the diagnostic trouble codes from memory and see if P0485 returns. If it does not, then the problem was most likely with the connections.
For this particular code, the most common area of concern is the relays and their connections.
If the code does return, you will need to test the fans and their associated circuits. Typically, there are 2 wires at each cooling fan in the module. First, disconnect the harness going to the cooling fan module. Using a Digital Volt Ohm Meter (DVOM), connect one lead of the meter to one terminal of one of the fans and connect the other meter lead to the other fan terminal. There should be no open or shorted circuit. Verify the resistance specifications for your specific vehicle. If the fan motor is either open or shorted (infinite resistance or 0 ohms), replace the cooling fan module or, if available, the cooling fan motor.
If that test passes, use a DVOM to check if you have 12V in the cooling fan module power supply circuit (Red lead to the fan power supply circuit, black lead to a good ground). Use a scan tool that can activate the cooling fans to turn them on. If there is no 12 volts to the fan, repair the wiring from the PCM or relay to the fan, or possibly replace a faulty PCM.
If that’s OK, check if you have a good ground at the cooling fan module. Connect a test light to the 12V battery positive (red terminal) and touch the other end of the test light to the ground circuit going to the cooling fan module circuit ground. Use the scan tool to activate the cooling fan module and see if the test light comes on each time the fan is activated. If the test light does not light up, there is a problem with the circuit. If it does light up, wiggle the wiring harness going to the fan to check for any flickering of the test light, indicating an intermittent connection.
If all tests have passed so far and you continue to get a P0485 code, it most likely indicates a failed cooling fan module, although a failed PCM cannot be ruled out until the fan is replaced. If unsure, seek assistance from a trained automotive diagnostician. PCMs must be programmed or calibrated to the vehicle in order to be installed correctly.
