B0040: Outside Temperature Sensor Circuit
The HVAC or body control module has detected an implausible signal from the outside (ambient) temperature sensor. The displayed outside temperature may be wrong or missing and automatic climate control may behave incorrectly.
What B0040 means
B0040 is a generic OBD-II body code in the airbags & restraints group. In plain English: the computer found an electrical fault in that circuit: an open, short or bad connection, or a failed component.
- Check engine light
- Usually off
- Emissions-related
- No
- Limp mode possible
- Not typical
- Drive cycle to clear
- Not required
Get it fixed
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Symptoms
- Warning light or symbol related to the affected system
- Comfort or safety function impaired
Common causes
- 01Ambient temperature sensor failedMost likely
- 02Wiring or connector fault to sensorPossible
Can I drive with B0040?
Generally yes.
This fault is unlikely to affect how the car drives. Fix it when convenient to restore the affected feature.
Why we rate it low
- Does not normally turn on the check engine light
Severity is our rule-based rating from the code’s flags in OBDex and the system it affects, not an inspection of your car. If the check engine light is flashing, or you notice overheating, low oil pressure, or brake or steering problems, stop driving and get help.
How to diagnose B0040
- 1
Confirm the code and save freeze-frame data
Read the codes with an OBD-II scan tool and note every code, not just B0040. Save the freeze-frame data (speed, load, temperature when the fault set) before clearing anything; it tells you when the fault happens.
- 2
Fix other faults first
If other codes are stored, start with power-supply, sensor-circuit and communication codes. They can cause this code as a side effect.
- 3
Test the circuit
Inspect the wiring and connector at the outside temperature sensor and hvac control module for damage, corrosion or chafing. With a multimeter, check supply voltage, ground and signal against the specifications in the service manual for your vehicle.
- 4
Test the likely causes in order
Start with the most likely cause and work down: Ambient temperature sensor failed; wiring or connector fault to sensor. Test before replacing parts.
- 5
Check for recalls and service bulletins
Look up recalls for your exact vehicle with its VIN, and ask a dealer about technical service bulletins (TSBs). Manufacturers sometimes issue software updates or extended warranties for known faults.
- 6
Clear the code and verify the repair
After the repair, clear the code and drive the car under the conditions in the freeze-frame data to confirm it does not return.
B0040 repair cost estimate
Estimate, not a quote. Labor times are typical ranges for this code from OBDex, and $125/hr is an assumed rate. Shop rates and labor times vary by vehicle and region. Diagnosis time may be billed separately.
- DIY difficulty
- Easy
- Good DIY job?
- Yes, with a scan tool
Basic hand tools; a common first DIY job.
Parts and systems involved
- Outside temperature sensor
- HVAC control module
- Body control module
B0040 FAQ
What does B0040 mean?
B0040 means “Outside Temperature Sensor Circuit”. The HVAC or body control module has detected an implausible signal from the outside (ambient) temperature sensor. The displayed outside temperature may be wrong or missing and automatic climate control may behave incorrectly.
Is B0040 serious? Can I keep driving?
We rate B0040 as low severity. Generally yes. This fault is unlikely to affect how the car drives. Fix it when convenient to restore the affected feature.
What causes B0040?
The most common causes, from most to least likely: Ambient temperature sensor failed; Wiring or connector fault to sensor. Test before replacing parts, because several causes can set the same code.
How much does it cost to fix B0040?
Typical labor is about $40–$125 at an assumed $125/hour shop rate, plus parts. That is an estimate, not a quote: labor times and rates vary by vehicle and region, and diagnosis may be billed separately.
Can I fix B0040 myself?
DIY difficulty is easy. Many owners can handle it with a scan tool, a multimeter and basic hand tools. Always confirm the cause before buying parts.
Will B0040 clear itself?
If the fault was intermittent, the warning may go off once the computer stops seeing it, but the code stays in memory. Fix the cause; clearing the code alone doesn’t repair anything.