What Does B2459 Mean?
This code indicates the Body Control Module (BCM) has detected an open circuit in the auxiliary heater overheat sensor monitoring system. The auxiliary heater (often a supplemental coolant heater or PTC heater in diesel vehicles and some hybrids/EVs) has a safety sensor to prevent overheating, and the BCM can no longer read its signal. This suggests a break in the wiring, a failed sensor, or a faulty connector preventing the safety monitoring system from functioning properly.
Common Causes
40%
Broken or corroded wiring in the auxiliary heater overheat sensor circuit
30%
Failed auxiliary heater overheat sensor (open internal circuit)
20%
Disconnected or loose connector at sensor or BCM
10%
Failed Body Control Module (BCM) internal circuit
Diagnostic Steps
1
Step 1: Perform visual inspection of the auxiliary heater assembly and locate the overheat sensor (typically a thermistor or thermal switch mounted on the heater housing). Check all connectors for corrosion, damage, or loose connections and verify connector terminals are properly seated.
2
Step 2: Using a digital multimeter, backprobe the overheat sensor connector at the sensor side and measure resistance across the sensor terminals. Compare readings to manufacturer specifications (typically 10-100k ohms at ambient temperature for thermistors, or continuity for thermal switches). An infinite resistance reading confirms sensor failure.
3
Step 3: Disconnect the sensor and measure continuity through the wiring harness from the sensor connector back to the BCM connector. Check for opens (infinite resistance) in both the signal and ground circuits. Also check for shorts to ground or power using the ohmmeter function.
4
Step 4: If wiring and sensor test good, suspect BCM failure. Verify BCM connector terminals are not pushed back or corroded. Clear codes, reconnect all components, and operate the auxiliary heater while monitoring live data to confirm the BCM now receives proper sensor signal and the code does not return.
5
Step 5: If repairs were made to wiring or sensor replacement was performed, cycle the auxiliary heater through several heat cycles and monitor for proper overheat protection functionality and verify no code recurrence.
Estimated Repair Cost
$150 - $650
Parts + labor, varies by vehicle and location
This code indicates the Body Control Module (BCM) has detected an open circuit in the auxiliary heater overheat sensor monitoring system. The auxiliary heater (often a supplemental coolant heater or PTC heater in diesel vehicles and some hybrids/EVs) has a safety sensor to prevent overheating, and t...
The most common cause of B2459 (Aux Heater Overheat Sensor Circuit Open) is: Broken or corroded wiring in the auxiliary heater overheat sensor circuit
Typical repair costs for B2459 range from $150 to $650, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Vehicle is safe to drive, but the auxiliary heater should not be used until repaired as the overheat protection safety system is non-functional. Continued operation of the auxiliary heater without overheat protection could result in heater damage or fire risk.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to B2459 to identify the root cause.
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Quick Info
Category
Body
System
Climate Control / Auxiliary Heating System
Difficulty
Type
Generic (SAE)
Recommended Tools
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