B0300 code meaning
HVAC Evaporator Temperature Sensor Circuit
What B0300 means
Public code lists do not show B0300, and factory documentation for it is missing too. The evaporator temperature sensor is a thermistor on or just behind the evaporator core; the climate controller cycles or throttles the compressor with it so that the core does not ice up (US6427465B1, US4326386A). Saab's 9-3 uses its own numbers: B0521 02 below 0.2 V and B0522 05 above 4.8 V at the module input, after which the outside temperature serves as a substitute value. HELLA reports that a badly sealed or drifting sensor can freeze the evaporator and choke the airflow. A circuit fault therefore tends to cost cooling comfort rather than leave the car without air.
B0300 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modeNot expected
- Repair difficultyMedium
- Typical labor0.5 to 4 hours
- Clears after repairWhen cleared with a scanner
Symptoms of B0300
- Cooling weak or fluctuating, especially in hot weather
- Airflow drops after a long drive because the evaporator ices up
Common causes of B0300
- Most likelyEvaporator thermistor open, shorted or out of tolerance
- PossibleSensor pushed out of the core or poorly sealed after service
- PossibleConnector at the HVAC housing loose or corroded by condensate
- Less likelySensor input in the climate control module faulty
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Evaporator temperature sensorHvac control module
What each part of B0300 means
| B | Body system: airbags, seat belts, power seats, lights and climate control. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 00 | The exact fault in that area. |
What to do next
- Note B0300 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about B0300
What does B0300 mean?
B0300 means "HVAC Evaporator Temperature Sensor Circuit". Public code lists do not show B0300, and factory documentation for it is missing too. It is a generic code, so it means the same on every make.
Is it safe to drive with B0300?
B0300 does not normally turn on the check engine light by itself. Watch for the symptoms above and have it checked soon.
Will B0300 fail an emissions test?
B0300 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is B0300 to fix?
Difficulty: medium. A typical repair takes 0.5 to 4 hours of work. This is a job for a workshop.
Will B0300 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.