Output air temperature sensor 3 circuit fault
What it means
B0509. Output air temperature sensor 3 circuit fault. The HVAC module cannot read the upper right duct discharge temperature and the body warning light comes on.. This Chevy-enhanced code applies to Chevrolet, GMC, Buick, Cadillac, Pontiac, Oldsmobile and Saturn models that use this code.
Findings, check these first
- Failed output air temperature sensor 3 in the upper right duct
- Open or short in the sensor 3 signal circuit
- Corroded sensor 3 connector
- Sensor 3 wiring chafed at the upper right duct
- Faulty sensor 3 input in the HVAC control module
Standard procedure
- Confirm the code with a scan tool
- Inspect output air temperature sensor 3 and its connector at the upper right duct
- Test the sensor 3 signal circuit for opens and shorts
- Compare live upper right duct temperature against a known good sensor
- Replace output air temperature sensor 3 only after circuit tests pass, then clear the code and retest
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Sources and review
Review: checked against the recorded source material.
- B0509 CHEVROLET Code: Meaning, Causes, and Symptoms | OBD2 Codes - OBD2 Codes
live reference page (obd2-codes.co)
Supports: CHEVROLET and GM-family meaning and likely causes for B0509
Fetched directly. B0509 reads "Output Air Temperature Sensor 3 Circuit".
- Code B0509 meaning and causes | OBDII.pro
live reference page (obdii.pro)
Supports: independent corroboration of the GM meaning
Fetched directly. obdii.pro is a separate publisher and gives the GM meaning as: Output air temperature sensor #3 (upper RH) circuit malfunction More details about the error B0509 and possible reasons for its occurrence.
Last updated 2026-10-10.
Report a correction for B0509. Include your vehicle, the issue and a source if you have one.