P0A57 – Battery Temperature Sensor “B” Circuit High

P0A57 means the Battery Management System (BMS) has detected a **high-voltage condition in Battery Temperature Sensor “B”**, one of the thermistors responsible for monitoring high-voltage battery module temperatures. A “Circuit High” fault almost always points to an **open circuit**, unplugged connector, broken wire, or a thermistor that has failed open internally. When the sensor reports impossibly low temperatures, the BMS cannot safely manage charging or cooling, so it restricts EV mode and regenerative braking. This guide explains the meaning of P0A57, the causes, symptoms, diagnosis, and repairs.

What Does P0A57 Mean?

P0A57 sets when the BMS detects **voltage from Battery Temperature Sensor “B” that is higher than the valid operating range**. Because HV battery temperature sensors are NTC thermistors, higher voltage means higher resistance — which the ECU interprets as very low temperature. An open circuit creates infinite resistance, causing maximum voltage and triggering the code.

This fault is electrical, not thermal. Unlike P0A56 (short/low voltage) or P0A55 (range/performance), P0A57 specifically indicates a **loss of continuity** in the Sensor “B” circuit.

Quick Reference

• OBD-II Family: P-Code (Hybrid/EV Battery System)
• Scope: Generic
• System: HV Battery Temperature Monitoring
• Difficulty Level: Moderate
• Estimated Repair Cost: €120–€450
• Last Updated: 2025-11-24

Real-World Example / Field Notes

P0A57 appears frequently on Toyota hybrids, Lexus RX/CT models, Hyundai/Kia HEVs, and GM/Voltec vehicles. On a Toyota Prius we serviced, Sensor “B” read –40°C due to a partially unplugged connector after a previous battery cleaning. On a Chevy Volt, corrosion inside the temperature harness created intermittent open circuits that only triggered P0A57 during cold starts. In both cases, regen was weak and EV mode was severely limited until the sensor circuit was repaired.

Symptoms of P0A57

• Very low or fixed temperature reading: Often –40°C (open-circuit default).
• Reduced EV power: BMS limits battery discharge.
• Weak or disabled regenerative braking: Regen is restricted.
• Battery cooling fan running excessively: System overcompensates for unknown temperature.
• Hybrid/EV warning messages: “Check Hybrid System,” “Battery Cooling,” etc.
• Engine running more frequently: EV-only mode may be unavailable.
• Limp mode in severe cases: Acceleration is limited to prevent battery stress.

Common Causes of P0A57

Most Common Causes

• Open circuit in Sensor “B” wiring.
• Disconnected or loose temperature sensor connector.
• Thermistor inside the battery module failed open.
• Broken wiring inside the HV battery harness.
• Corroded terminals preventing proper continuity.

Less Common Causes

• Faulty input channel inside the battery ECU.
• Moisture intrusion causing terminal separation.
• Improper repairs after battery servicing.
• Pin deformation inside the battery connector housing.

Diagnosis: Step-by-Step Guide

Your goal is to confirm whether Sensor “B” has an open circuit, poor connection, or internal thermistor failure.

Tools You’ll Need: Hybrid-safe scan tool, multimeter, insulated gloves, wiring diagram, resistance chart, optional thermal camera.

1. Check temperature readings for Sensor “B.” A fixed –40°C value strongly indicates an open circuit.
2. Inspect the connector. Look for loose pins, corrosion, water droplets, or damaged locking tabs.
3. Perform a wiggle test. If the reading changes, you’ve found a broken wire or bad pin fit.
4. Measure signal voltage. Near reference voltage = open circuit.
5. Test thermistor resistance. Infinite resistance confirms an open sensor.
6. Check continuity between sensor and ECU. Identify wiring breaks or high-resistance segments.
7. Inspect for corrosion inside the battery pack. Oxidized sensor pins are a frequent cause of opens.
8. Check cooling airflow. Poor airflow can cause condensation, leading to connector issues.
9. Review freeze-frame data. Open circuits often occur at startup or after hitting bumps.
10. Check for related cooling system or battery faults. These can help narrow down the root cause.

Pro Tip: When a temperature sensor reports exactly –40°C in READY mode, the circuit is almost certainly open — the hallmark of a P0A57 fault.

Possible Fixes & Repair Costs

• Reconnect or reseat the sensor connector: €0–€20.
• Repair open or broken wiring: €20–€120.
• Clean or replace corroded terminals: €10–€50.
• Replace Battery Temperature Sensor “B” (if separate): €150–€300.
• Replace the battery ECU (if input circuit is damaged): €200–€450.
• Replace HV battery module containing the thermistor (sealed designs): €250–€600+.

Open circuits and loose connectors are far more common than actual battery module failures. Always test continuity before replacing high-cost components.

Can I Still Drive With P0A57?

You can usually drive, but EV mode and regen will be significantly limited. The BMS restricts battery load because it cannot verify pack temperature. Avoid heavy acceleration, long EV-only driving, steep hills, or towing. If the vehicle enters limp mode or displays critical battery warnings, stop driving immediately.

Last updated: January 22, 2026