B2A0813 - B2A0813 Evaporator Outlet Refrigerant Temperature Sensor Open Circuit

Fault Definition

DTC B2A0813 belongs to the automobile air conditioning control system diagnostic trouble code; its core meaning points to an open circuit condition in the evaporator outlet refrigerant temperature sensor (NTC thermistor) circuit. In the vehicle electrical architecture, this signal serves as critical data for a simulated feedback loop and is used by the right domain controller for real-time monitoring of the evaporator's physical temperature and cooling efficiency. When the system detects the voltage signal returned by the sensor rises abnormally above a specific threshold, it is judged as an "open circuit" fault. This definition excludes simple resistance drift or short circuit situations, clearly pointing to signal level loss of control caused by circuit connection interruption, directly affecting the automatic regulation strategy execution of the air conditioning system.

Common Fault Symptoms

After this fault code is set, the vehicle control logic enters Failsafe Mode; owners driving may observe the following specific feedback:

• Reduced Cooling Efficiency: Since the system cannot acquire accurate evaporator outlet temperature data, the compressor or expansion valve execution logic will be restricted, causing partial HVAC system failure, hindering adjustments to air volume or refrigerant circulation.
• Instrument Panel Status Light Indications: Some models may display "Check Vehicle" or specific HVAC system warning icons on the instrument panel, reminding drivers that the current mode is limited.
• Inaccurate Temperature Control: The actual interior temperature cannot be maintained at the set value through closed-loop control of the automatic air conditioning system, resulting in fluctuating temperatures or failure to cool.

Core Fault Cause Analysis

Based on the logical breakdown of diagnostic data, the physical root causes leading to DTC B2A0813 are mainly summarized into three dimensions:

• Hardware Component Failure: The internal sensing element of the evaporator outlet refrigerant temperature sensor breaks, damages, or loses insulation performance completely, failing to form a normal resistor voltage divider network.
• Wiring/Connector Connection Abnormality: There is high impedance open circuit (such as metal fatigue fracture) inside the wire harness or connector pins receding/bucking off, oxidation, etc., making physical conductivity between sensor signal line and controller interrupted, causing the signal line to float.
• Controller Logic Computation Fault: The right domain controller's analog input module (ADC) itself suffers hardware damage, leading to incorrect voltage value reading, unable to correctly identify normal resistor voltage divider signals, falsely reporting an open state.

Technical Monitoring & Trigger Logic

Diagnostic algorithms integrated inside the control unit conduct dynamic real-time monitoring of this sensor loop; specific judgment process is as follows:

• Monitoring Target Parameters:
  • Core indicator is the output end voltage (Signal Voltage) of the evaporator outlet refrigerant temperature sensor.
  • Under normal conditions, voltage usually maintains a low resistor voltage divider value range; the key for fault judgment lies in the degree of deviation from abnormal voltage.
• Trigger Threshold Range:
  • The critical condition for judging open circuit is that the sensor output voltage is higher than $4.95V$.
  • When monitored voltage exceeds this value, it indicates the circuit is in a high impedance or floating state, meaning "open circuit" in physical sense.
• Specific Condition Judgment:
  • Fault judgment trigger condition is limited to dynamic monitoring period after start switch placed in ON gear (ignition system powered).
  • System needs continuous sampling during vehicle operation; once satisfying above voltage threshold conditions and lasting for timer-set time, it is confirmed as valid fault and DTC B2A0813 recorded.