P2E78

Code

Generic P — Powertrain

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

Brand: Generic

Type: P — Powertrain

Views: UK: 21 EN: 26 RU: 21

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Page language: EN

Causes

Intermittent open or short in sensor D wiring (chafing, broken conductor, intermittent connector pin contact)
Corroded, loose or contaminated connector at the sensor or control module
Faulty battery pack coolant temperature sensor D (internal intermittent thermistor)
Water intrusion or contamination in connector or harness
Intermittent reference voltage/ground fault from control module or intermediate junction
Aftermarket repairs or modifications that disturbed wiring

Symptoms

Malfunction Indicator Lamp (MIL) or battery/thermal management warning lamp illuminated
Intermittent or incorrect battery coolant temperature readings on scan tool/live data
Battery thermal management may enter fault/derate mode or run cooling/heating incorrectly
Reduced charge/discharge power or altered charge strategy
Possible intermittent fan/pump activation or abnormal HVAC behavior
Sometimes no drivability symptoms except stored fault if condition is intermittent

What to check

Follow all high-voltage safety procedures before working on the battery pack or connectors; isolate high-voltage system and use PPE as required
Read and record freeze frame and freeze data for P2E78 using a capable scan tool
Retrieve related DTCs (other sensor circuits, communication errors, CAN bus faults)
Visually inspect sensor D connector, wiring, and routing for chafing, pin corrosion, water ingress, or damage
With vehicle and HV system made safe, backprobe sensor D connector and monitor live PID for that sensor while gently wiggling harness and connector to reproduce the fault
Check reference voltage and ground at the sensor connector with scan tool PIDs or multimeter (do not disconnect while powered unless following manufacturer procedures)

Signal parameters

Sensor type: NTC thermistor (resistance decreases as temp rises)
Expected output: variable voltage signal to BMS/TCU (typical range 0.1–4.8 V depending on temperature and vehicle design)
Typical mid‑range: ~1.5–3.0 V at ~20–25 °C (manufacturer-specific)
Resistance: should change smoothly with temperature (typical order of magnitude at 25 °C often 1–20 kΩ depending on sensor — consult vehicle-specific spec)
Fault conditions: open circuit (very high resistance/OL) or short to ground (near 0 Ω) or intermittent/noisy voltage steps/volatility

Diagnostic algorithm

Confirm DTC and capture freeze frame and live data. Note when code sets (key on, driving, charging, ambient conditions).
Review manufacturer wiring diagram to identify sensor D circuit: reference voltage, signal, and ground paths and any intermediate connectors or junctions.
Visually inspect sensor D connector and harness for damage, corrosion, coolant intrusion, or pin movement. Repair any obvious issues.
With HV system made safe, disconnect the sensor and measure sensor resistance at two known temperatures (ambient and warmed) and verify resistance changes smoothly. Replace sensor if internal intermittent behavior is found.
With harness connected and vehicle in a safe state per manufacturer, backprobe the signal, reference and ground while monitoring live data on a scan tool. Wiggle the harness and connector to try to reproduce intermittent behavior.
Measure reference voltage and ground integrity at the connector. Verify stable reference (usually 5 V or specified value) and solid ground. Repair/replace wiring/joints if unstable.
Perform continuity and insulation tests between sensor connector and control module pins to find intermittent opens or shorts. Repair any damaged wires, connectors or corrosion.
If wiring and connector check good, swap sensor D with an identical known-good sensor (if available) or replace sensor and retest.
If intermittent persists after sensor and harness repair, inspect control module connector and ground. Consider module input circuit testing or replacement per manufacturer guidance.
After repair, clear codes, perform functional test under the same conditions that previously set the code, confirm stable readings and road/charge cycle verification. Document repair and retest.

Likely causes

Intermittent connector contact or corrosion at sensor D (most common)
Broken or chafed wire in harness to sensor D (wiggle-sensitive)
Faulty sensor D (thermistor intermittent internally)
Contamination or coolant leak into connector causing intermittent readings
Intermittent ECU/PCM input problem (least common)

Fault status

⚠️ Status

Intermittent/erratic signal detected from Battery Pack Coolant Temperature Sensor D circuit — intermittent open/short, poor connection, or sensor fault suspected.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours