B1849

Code

Other B — Body

Climate Control Temperature Differential Circuit Failure

Brand: Other

Type: B — Body

Views: UK: 17 EN: 27 RU: 20

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HVAC automatic temperature control behaves incorrectly (incorrect blends or mode changes)
Incorrect inside/outside temperature readings on the HVAC display
Reduced HVAC performance or inability to maintain set cabin temperature
DTC B1849 stored and possibly a warning/message on the dash or HVAC display
Intermittent or permanent failure of automatic climate functions (recirculation, AC cycling)

Read and record all stored DTCs and freeze-frame data with a scan tool
Inspect sensor connectors and wiring for corrosion, damage, or water intrusion
Verify HVAC module power, ground, and CAN/data connections
Check for aftermarket devices or recent repairs that may have disturbed wiring
Measure sensor resistance/voltage at the connector and at the module to isolate a wiring fault
Perform wiggle test on wiring while monitoring live data for intermittent faults

Typical sensor type: NTC thermistor — resistance decreases as temperature rises (examples vary by vehicle)
Expected sensor voltage (sensor divider): commonly ~0.5–4.5 V depending on temperature and design
Open-circuit condition: sensor voltage may be at supply rail (near 5 V) or readings out of expected temperature range
Short-to-ground: sensor voltage near 0 V
Resistance examples (generic NTC): ~1 kΩ–100 kΩ across -40°C to +125°C (consult OEM spec for exact values)
Module-reported differential temperature: expected small positive/negative value within a few °C when sensors are functioning

Retrieve B1849 and any related codes with a capable scan tool; record freeze frame and live sensor values.
Visually inspect the sensor(s), connector(s), and wiring harness for damage, corrosion, or water ingress.
Identify the specific sensors involved (interior/exterior or evaporator/ambient) from OEM wiring diagrams.
With ignition on, backprobe the sensor connector and measure signal voltage and reference voltages at the sensor; compare to OEM spec.
Measure sensor resistance with sensor disconnected and ambient temperature known; compare to spec chart. Check for open/short.
Check continuity and resistance of the signal, ground and supply wires from sensor to HVAC module; repair any open/shorts.
Inspect and verify HVAC module power and ground integrity; check for proper CAN/data communication if sensors report via network.
If wiring and supply/ground are good but sensor is out of spec, replace the temperature/differential sensor and retest.
If sensor and wiring are good but fault persists, consider HVAC/climate control module diagnosis—check software updates and module bench tests as per OEM procedures.
Clear codes, perform functional test of climate system and a road/soak test to verify fault does not return.