Code Meaning
P0A39 indicates a problem with the hybrid engine’s temperature sensor #2, which monitors the temperature of the transaxle fluid. This sensor uses a thermistor: its electrical resistance varies with the fluid temperature. The hybrid (HV) ECU uses this data to:
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Limit engine load and prevent overheating.
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Detect sensor or wiring malfunctions.
Note: Its operation is identical to temperature sensor #1, but it is dedicated to the transaxle.
Visible Symptoms
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🔥 Check engine light on (or “Service hybrid system soon” message).
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No other obvious engine symptoms (the system automatically limits performance to protect the engine).
Probable Causes
| Priority | Cause | Description |
|---|---|---|
| 1 | Electrical Issues | Open/short-circuited wiring harness, oxidized/faulty connectors, bent/broken pins. |
| 2 | Sensor Failure | Thermistor out of specifications (incorrect resistance). |
| 3 | HV ECU Failure | Rare – check after eliminating other causes. |
| 4 | Faulty Hybrid Engine | Very unlikely (the code specifically targets the sensor circuit). |
Diagnostic Procedure
Required Tools: Digital multimeter, OBD2 scanner, vehicle technical documentation.
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Visual Inspection (80% of solutions):
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Check the sensor wiring harness (physical damage, rodent bites).
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Inspect the connectors:
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Bent, broken, or corroded pins.
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Loose connection or moisture.
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Clean with contact spray if necessary.
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Sensor Electrical Test:
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Disconnect the connector and measure the resistance between the pins:
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Expected Value: Varies with temperature (refer to manufacturer documentation).
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Anomaly: Infinite resistance (open circuit) or zero (short circuit).
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Circuit Test to HV ECU:
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Use the multimeter in continuity mode:
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Check for no break between the sensor and the ECU.
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Measure the supply voltage (usually 5V) with the ignition on.
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ECU Diagnostic:
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If the wiring and sensor are intact, suspect an HV ECU failure (use a scanner to check live data).
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Common Repairs
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🔧 Replacement of temperature sensor #2 (moderate cost).
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🔧 Repair of wiring harness/connector (soldering, plug replacement).
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💡 Tip: After repair, clear the code and test the vehicle under real conditions (temperature rise).
⚠️ Hybrid Safety: Always shut off the high voltage (follow manufacturer procedures) before any intervention.
Why This Approach?
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Electrical causes (wiring, connectors) account for 90% of P0A39 cases.
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The sensor is inexpensive and easy to test – replacing it is a quick solution if resistance values are inconsistent.
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The ECU or hybrid engine is rarely at fault (replace only as a last resort).
Source: Manufacturer technical data (Toyota/Lexus, Honda, Ford Hybrid).