P0A01 Range / Performance

Possible Causes

  • Failure of the engine coolant temperature sensor.
  • Open or short circuit in the electrical wiring of the coolant temperature sensor (related to engine electronics).
  • Poor electrical connection in the coolant temperature sensor circuit (related to engine electronics).

Meaning of Code P0A01
This code indicates a malfunction detected in the high voltage (HV) coolant temperature sensor circuit, essential to the inverter with converter cooling system.

Initial Diagnosis of Code P0A01

  1. Review the causes mentioned above.
  2. Visually inspect the electrical wiring and connectors associated with the sensor.
  3. Check for damaged components, corroded, broken, deformed, or improperly engaged connector pins.

Possible Symptoms

  • Illumination of the Malfunction Indicator Lamp (MIL) or engine warning message.

Detailed Description
The Power Control Unit (ECU) monitors the HV coolant temperature via a sensor integrated into the inverter with converter. This sensor allows the ECU to:

  • Assess the efficiency of the inverter cooling system.
  • Limit the inverter output power in case of overheating to protect the system.
  • Detect anomalies in the sensor or its circuit (inconsistent values, interruptions, short circuits).

The inverter with converter specifically identifies faults related to the HV temperature sensor and its wiring. A P0A01 code is then recorded, prompting a thorough check of the electrical components and associated connections.

Note: Neglecting to resolve this fault may lead to limited vehicle performance or damage to the inverter.

P0A02 Low Engine Electronics Coolant Temperature Sensor Circuit

P0A02 Possible Causes

  • Faulty engine coolant temperature sensor
  • Engine electronics – Coolant temperature sensor harness open or short circuit
  • Engine electronics – Coolant temperature sensor circuit poor electrical connection

What Does It Mean?

First Diagnose P0A02 Code

Check the “possible causes” listed above. Visually inspect the related cable harness and connectors. Check for damaged components and look for broken, bent, pushed out, or corroded connector pins.

P0A02 Possible Symptoms

Engine light on (or service engine soon warning light)

P0A02 Description

The Power Management Control Electronic Control Unit (ECU) detects the HV coolant temperature using the coolant temperature sensor integrated in the inverter with converter. The Power Management Control ECU uses the coolant temperature sensor signals to verify the effectiveness of the inverter cooling system. If necessary, the Power Management Control ECU will limit the inverter output to prevent inverter overheating. The Power Management Control ECU also detects sensor malfunctions based on HV coolant temperature sensor values. The inverter with converter assembly detects malfunctions in the HV coolant temperature sensor and its wiring.

P0A03 High Engine Electronics Coolant Temperature Sensor Circuit

P0A03 Possible Causes

  • Faulty engine coolant temperature sensor
  • Engine coolant temperature sensor harness is open or shorted
  • Engine coolant temperature sensor circuit poor electrical connection

What Does It Mean?

Check the “Possible Causes” listed above. Visually inspect the related wiring harness and connectors. Check for damaged components and look for broken, bent, pushed out, or corroded connector pins.

P0A03 Possible Symptoms

Engine Light On (or Service Engine Soon Warning Light)

P0A03 Description

The Power Management Control Electronic Control Unit (ECU) detects the HV coolant temperature using the coolant temperature sensor integrated in the inverter with converter assembly.

The Power Management Control ECU uses the coolant temperature sensor signals to verify the effectiveness of the inverter cooling system.

If necessary, the Power Management Control ECU will limit the inverter output to prevent inverter overheating. The Power Management Control ECU also detects sensor malfunctions based on HV coolant temperature sensor values. The inverter with converter assembly detects malfunctions in the HV coolant temperature sensor and its wiring.
OBDII Code P0A03 – Engine Electronics Coolant Temperature Sensor Circuit High

P0A04 Intermittent Engine Coolant Temperature Sensor Electronic Circuit

P0A04 Possible Causes

  • Faulty motor electronics coolant temperature sensor
  • Motor electronics coolant temperature sensor harness open or short circuit
  • Motor electronics coolant temperature sensor circuit poor electrical connection

What Does It Mean?

First Diagnose P0A04 Code

Check the “possible causes” listed above. Visually inspect the related cable harness and connectors. Check for damaged components and look for broken, bent, pushed out, or corroded connector pins.

P0A04 Possible Symptoms

Engine light on (or service engine soon warning light)

P0A04 Description

The Power Management Control Electronic Control Unit (ECU) detects the HV coolant temperature using the coolant temperature sensor built into the inverter with converter. The Power Management Control ECU uses signals from the coolant temperature sensor to verify the efficiency of the inverter cooling system. If necessary, the Power Management Control ECU will limit the inverter output to prevent inverter overheating. The Power Management Control ECU also detects sensor malfunctions based on HV coolant temperature sensor values. The inverter with converter assembly detects malfunctions in the HV coolant temperature sensor and its wiring.

P0A05 Engine Coolant Pump «A» Control Electronic Circuit / Open

P0A05 Possible Causes

  • Faulty cooling pump “A”
  • Coolant pump wiring harness is open or short-circuited
  • Poor electrical connection in the coolant pump circuit

What Does It Mean?

First Diagnose Code P0A05

Check the “possible causes” listed above. Visually inspect the wiring harness and associated connectors. Check for damaged components and look for broken, bent, pushed out, or corroded connector pins.

P0A05 Possible Symptoms

Check engine light illuminated (or service engine soon warning light)

P0A05 Description

The Engine Control Module (ECM) monitors the engine electronics cooling pump. The ECM sets the OBDII code when the engine electronics cooling pump does not meet factory specifications.

P0A06 Coolant Pump “A” Control Circuit Low

P0A06 Possible Causes

  • Faulty cooling pump “A”
  • Coolant pump wiring harness is open or short-circuited
  • Poor electrical connection in the coolant pump circuit

What Does It Mean?

Initial Diagnosis of Code P0A06

Check the “possible causes” listed above. Visually inspect the wiring harness and associated connectors. Look for damaged components and check for broken, bent, pushed out, or corroded connector pins.

P0A06 Possible Symptoms

Check engine light illuminated (or service engine soon warning light)

P0A06 Description

The Engine Control Module (ECM) monitors the engine electronics cooling pump. The ECM sets the OBDII code when the electronic engine cooling pump does not meet factory specifications.

P0A07 Engine Electronics Coolant Pump “A” Control Circuit

P0A07 Possible Causes

  • Faulty cooling pump “A”
  • Coolant pump wiring harness is open or short-circuited
  • Poor electrical connection in the coolant pump circuit

What Does It Mean?

First Diagnose Code P0A07

Check the “possible causes” listed above. Visually inspect the wiring harness and associated connectors. Check for damaged components and look for broken, bent, pushed out, or corroded connector pins.

P0A07 Possible Symptoms

Check engine light illuminated (or service engine soon warning light)

P0A07 Description

The Engine Control Module (ECM) monitors the engine electronics cooling pump. The ECM sets the OBDII code when the electronic engine cooling pump does not meet factory specifications.

P0A08 DC/DC Converter Status Circuit

P0A08 Possible Causes

  • Cable harness or connector
  • Water pump assembly
  • Cooling system
  • Hybrid vehicle transaxle assembly
  • Generator cable
  • Motor cable
  • Frame wire
  • Inverter with converter
  • Fuse junction block assembly (MAIN)
  • Fuse (DC/DC-S, IGCT, IGCT No. 2)
  • Fuse junction block assembly (DC/DC)

What Does It Mean?

First Diagnose Code P0A08

Check the “Possible Causes” listed above. Visually inspect the cable harness and associated connectors. Check for damaged components and look for broken, bent, pushed out, or corroded connector pins.

P0A08 Possible Symptoms

Engine light on (or service engine soon warning light)

P0A08 Toyota Description

The hybrid vehicle converter (DC/DC converter) converts the 201.6 V DC from the HV battery into 12 V DC to power areas such as vehicle lighting, audio, and ECU systems. Additionally, it charges the auxiliary battery. A transistor bridge circuit initially converts 201.6 V DC into alternating current, and a transformer steps down its voltage. Then, it is rectified and smoothed (into DC) and converted into 12 V DC. The hybrid vehicle converter controls the output voltage to maintain a constant voltage across the auxiliary battery.

The Power Management Control ECU (HV CPU) uses the NODD signal line to send a shutdown command to the hybrid vehicle converter and receive signals indicating the normal or abnormal status of the 12 V charging system. If the vehicle is driven with an inoperative hybrid vehicle converter, the auxiliary battery voltage will drop, preventing continuous vehicle operation. Therefore, the Power Management Control ECU (HV CPU) monitors the operation of the hybrid vehicle converter and alerts the driver if it detects a malfunction.

P0A09 DC/DC Converter Status Circuit Low

P0A09 Possible Causes

  • Faulty DC/DC converter (internal failure, overheating, damaged electronic components).
  • DC/DC converter wiring harness open or short-circuited (wires damaged by rubbing, rodents, or exposure to extreme temperatures).
  • Poor electrical connection in the DC/DC converter circuit (oxidation, loose connectors, broken solder joints).
  • Faulty Powertrain Control Module (PCM) (outdated software, hardware failure).
  • Low or unstable battery voltage (affects converter operation).
  • Faulty fuse or relay in the converter power circuit.
  • Electromagnetic interference disrupting signals between the PCM and converter.

What Does It Mean?
This code indicates a malfunction in the communication or operation of the DC/DC converter, a key component especially in hybrid or electric vehicles. This converter manages the voltage between the high-voltage battery (e.g., 400V) and the 12V system, powering accessories and the auxiliary battery. If the PCM detects an anomaly via the feedback signal, it stores code P0A09, risking the deactivation of critical power functions or limiting performance.

First Diagnose Code P0A09

  1. Use an advanced diagnostic tool: Check live data (e.g., converter input/output voltage, fault signal status).
  2. Check the 12V and high-voltage battery voltage (if applicable) with a multimeter.
  3. Test the DC/DC converter: Measure output voltage (should be ~14V when operating) and check for overheating.
  4. Inspect associated fuses and relays (refer to the workshop manual to locate components).
  5. Look for PCM software updates (some faults are resolved by recalibration).
  6. Perform a circuit load test to identify abnormal resistances or current losses.

P0A09 Possible Symptoms

  • Check engine light on (or service engine soon warning light).
  • 12V battery not charging (dim headlights, hard starting).
  • Electrical systems deactivated (air conditioning, power steering).
  • Reduced performance (limp mode activated by the PCM).
  • Abnormal noises (buzzing or clicking from the converter).

P0A09 Description
The PCM regulates the DC/DC converter via a PWM (pulse width modulation) signal to adjust the output voltage according to the vehicle’s needs (e.g., charging the 12V battery, powering equipment). The converter returns a fault signal in case of:

  • Over/under-voltage (e.g., >16V or <10V output).
  • Internal overheating (built-in temperature sensor triggers an error).
  • Communication fault (CAN bus signal interruptions).
    An unresolved P0A09 can damage the 12V battery or disrupt electronic modules (e.g., regenerative braking controller).

Recommended Repairs

  • Replacement of the DC/DC converter (check compatibility with the model).
  • Repair of damaged wiring harness and add anti-abrasion protection.
  • Cleaning connectors with electrical contact spray.
  • Updating PCM firmware or replacement if faulty.
  • Estimated cost: €200 to €1500 depending on the cause (parts and labor).

This addition clarifies the technical implications, detailed diagnostic steps, and practical consequences for the driver.

P0A0A High Voltage System Interlock Circuit

P0A0A Possible Causes

  •     Faulty high voltage system interlock
  •     High voltage system interlock circuit open or shorted
  •     Poor electrical connection in high voltage system interlock circuit

What Does It Mean?

Initial Diagnosis of P0A0A Code

    Check the “possible causes” listed above. Visually inspect related wiring harnesses and connectors. Check for damaged components and look for broken, bent, pushed out, or corroded connector pins.

P0A0A Possible Symptoms

    Illuminated check engine light (or service engine soon warning light)

P0A0A Description

The diagnostic trouble code (DTC) monitors the detected voltage when the control voltage is high to determine if the circuit is faulty.
OBDII Code P0A0A – High Voltage System Interlock Circuit