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DTC codes / P2Exx · Hybrid propulsion / P2E78

P2E78Hybrid/EV Battery Pack Coolant Temperature Sensor D Circuit Intermittent/Erratic

P2E78 indicates that the hybrid or electric vehicle battery pack coolant temperature sensor D circuit is reporting an intermittent or erratic signal to the battery energy control module. The computer has detected that the sensor's voltage or resistance readings are jumping unpredictably or dropping out momentarily, rather than providing a stable temperature measurement.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaHybrid propulsion

Symptoms you may notice

  • Illumination of the check engine light or a dedicated hybrid system warning light.
  • Often no symptoms besides the warning light, especially if the erratic signal is brief.
  • Reduced hybrid system power or a forced limp mode if the fault triggers protective strategies.
  • Inconsistent or disabled battery cooling fan operation, leading to elevated battery temperatures.
  • In rare cases, a sudden loss of propulsion power or a message to pull over safely.
  • Poor fuel economy or increased engine running time if the hybrid system limits electric-only operation.

Common causes, most likely first

  1. A faulty coolant temperature sensor D, such as an internal short or open circuit that intermittently appears with heat or vibration.
  2. Damaged or corroded wiring harness between the sensor and the battery energy control module, often due to chafing or rodent damage.
  3. Loose or contaminated electrical connector pins at the sensor or module, causing an intermittent connection.
  4. Coolant level that is too low or air pockets in the battery cooling circuit, causing the sensor to read erratic temperatures.
  5. A failing battery energy control module or a software glitch in its signal processing circuit.
  6. Electrical interference from nearby high-voltage cables or aftermarket accessories affecting the sensor signal.
  7. A stuck or malfunctioning coolant flow valve that causes temperature fluctuations the sensor correctly reports as erratic.

How to diagnose and fix it

  1. Perform a visual inspection of the sensor, its connector, and the wiring harness for obvious damage, corrosion, or loose pins.
  2. Check the battery coolant level and bleed the cooling system if air pockets are suspected, following the manufacturer's procedure.
  3. Use a scan tool to monitor the sensor D temperature reading live while wiggling the harness and connector to reproduce the erratic signal.
  4. Measure the sensor's resistance or voltage at the connector and compare it to the manufacturer's specification for the current temperature.
  5. If the sensor is out of specification or its signal jumps during a wiggle test, replace the sensor and clear the code.
  6. If the sensor and wiring test good, check for continuity and short-to-ground or short-to-power on the signal circuit between the sensor and the control module.
  7. As a last step, reflash or replace the battery energy control module only after all other causes have been ruled out and technical service bulletins have been checked.

Before you replace the battery coolant sensor

  • A low coolant level or trapped air in the battery cooling loop is a common cause of erratic temperature readings that mimic a failed sensor, so always verify coolant condition and bleed the system first.
  • The code can be set by a poor connection at the sensor connector that only acts up when the vehicle is moving or when the harness is exposed to heat, so a static resistance check may pass while the fault is still present.
  • On many hybrids, the battery cooling system shares components with the cabin air conditioning or a dedicated chiller, and a fault in that shared circuit can produce temperature swings that the sensor faithfully reports.
  • Replacing the sensor without checking the control module's ground and reference voltage can lead to a repeat failure if the module itself is intermittently dropping the signal.
  • Intermittent codes often clear themselves after a few drive cycles, but the underlying wiring damage or coolant issue will remain and can cause a more severe fault later.

Can I keep driving with P2E78?

You can generally continue driving if the only symptom is a warning light and the vehicle maintains normal power, but avoid aggressive driving or heavy electrical loads that stress the hybrid battery. If the vehicle enters limp mode, shows a high battery temperature warning, or loses power, stop safely and have it towed to avoid damaging the high-voltage battery.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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