Code Definition

B1021 — Air thermo sensor system Circuit shorted or blower fan switch malfunction

System: Body  |  Type: Manufacturer-Specific  |  Make: Mitsubishi

About B1021

B1021 is a manufacturer-specific body diagnostic trouble code recorded for Mitsubishi vehicles. It belongs to the B-prefix family of codes, which covers body, interior, airbag, or comfort-electronics faults.

When B1021 is stored, the module has detected a short-circuit (voltage pulled to ground or to supply) in the monitored circuit; a signal or supply voltage lower than the expected operating range; and a sensor-related fault — the reported value from the named sensor is out of spec.

Scanning & clearing: Body (B) codes are often stored in the body control module, airbag module, or climate module. A basic OBD-II tool usually will not read body codes — an enhanced or OEM-level scan tool is typically required.

B1021 is a manufacturer-specific code. The same code string on a different make may have a different meaning. Always confirm the code's definition against the vehicle's service information for the specific make and model under test.

Code Information

On Mitsubishi vehicles with automatic climate control, the HVAC control module uses an in car (cabin) air thermo sensor to measure ambient interior temperature and an input from the blower fan switch to determine requested fan behaviour. The module applies a 5V reference or regulated voltage to the sensor and reads back a varying voltage based on a negative temperature coefficient (NTC) thermistor. B1021 sets when the module detects the air thermo sensor circuit voltage is pegged low in a manner consistent with a short circuit, or when the blower fan switch input is stuck or electrically inval...

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