Fault code P0A05 – motor electronics coolant pump “A” control circuit/open

Fault code P0A05 is called “Motor Electronics Coolant Pump “A” Control Circuit/Open” but in different programs it may be called differently. This fault designation applies to all vehicles equipped with OBD-II.

Technical description and explained code P0A05

Fault code P0A05 is intended to be displayed only on hybrid vehicles. Indicates that the Powertrain Control Module (PCM) has detected an open circuit in the coolant pump electronics. It refers to the electronic motor unit designated by the letter “A”.

Fault code P0A05 – motor electronics coolant pump "A" control circuit/open

The inverter cooling pump transmits the speed data to the power distribution ECU (Hybrid Master Processor). The power distribution ECU (hybrid master processor) monitors the speed and detects faults.

This cooling system is self-contained and independent of the engine cooling system. The hybrid system control module fully monitors the electric pump, fan, and cooling system and records malfunctions.

If the Hybrid Control Module detects an unacceptable speed or intermittent stopping of the inverter cooling system pump, it will save code P0A05. The Malfunction Indicator Light (MIL) may then illuminate.

For most vehicles, it will take several failure cycles before the warning lamp will illuminate.

Symptoms of vehicle malfunctions

The main signal that an error P0A05 has occurred is the Malfunction Indicator Lamp (MIL) is also known as the CheckEngine Light.

It can also be warning signs such as:

  1. The “Check engine” control lamp on the control panel will light up (the code will be stored as a fault).
  2. Other related fault codes may be present.
  3. Severe heating.
  4. Reduced vehicle performance.
  5. Emergency engine shutdown due to inverter overheating.

The severity of fault code P0A05 is high because there is a risk of excessive temperature rise when the cooling system malfunctions. This, in turn, may cause the engine to shut down in an emergency.

Factors that can cause this error code

The error code P0A05 can mean that one or more of the following problems have occurred:

  • Malfunction of the cooling system of the inverter.
  • Corrosion, loose or broken electrical wiring.
  • Poor electrical connection of connectors.
  • Faulty inverter cooling system pump assembly.
  • Sometimes faulty hybrid system module is the cause.

How to fix or reset OBD-2 code P0A05

Some suggested steps for troubleshooting and fix the error code P0A05:

  1. Connect an OBD-II scanner to the vehicle’s diagnostic connector and read all stored data and error codes.
  2. Clear the error codes from the computer memory and test-drive the vehicle to see if the P0A05 code appears again.
  3. If the error code appears again, visually inspect the electrical wires and connectors related to the inverter cooling system.
  4. If necessary, repair or replace any shorted, broken, damaged, or corroded components.
  5. Check the amount of coolant in the hybrid system.
  6. Check the coolant hose.
  7. Check the inverter cooling system pump.
  8. Test the battery and make sure the system voltage is normal.
  9. If the problem persists, check and replace the hybrid system control module if necessary.

Diagnose and repair of problems

The first step in the P0A05 troubleshooting process is to review the Technical Service Bulletins (TSB). For known problems with a particular vehicle.

The advanced diagnostic steps are very vehicle specific. They may require, special equipment and knowledge to accurately perform the steps.

Visual Inspection

Perform a visual inspection of the inverter cooling system, check the coolant level. Inspect the hoses, they should not be pinched or clogged and there should be no cuts, tears or other damage.

If the system is checked and is in good condition, the cooling pump should be checked. Most often it is a failed pump that causes fault P0A05.

Check the electrical connections of the inverter cooling system. Disconnect the wiring connectors and inspect for corrosion or bent contacts. If necessary, repair and apply dielectric grease to the terminals.

Also, check the operation of the cooling fan and for dirt and dust between the engine cooler, air conditioner, and inverter radiators. If the radiator cells are clogged, the fan will not be able to handle cooling efficiently.

If these steps could not help find the problem, there may be a bad connection to the hybrid system module or the module itself is defective. In this case, it is better to consult a specialist for reprogramming or replacing the control module.

On which vehicles does this problem occur most frequently

Fault code P0A05 can occur on different vehicles but there are statistics on which brands this occurs most often. Here is a list of some of them:

  • Chevrolet
  • Ford
  • Lexus (rx400h)
  • Mercedes-Benz
  • Nissan
  • Opel
  • Toyota (Highlander)

Fault code P0A05 can sometimes be found with other errors. The most common are the following: P0A06, P0A07, P0C73.


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