Air Quality Sensor (RPO K14)

WARNING: This page is about a different car, the 2008 Cadillac CTS. However, it is still accessible from the selected car via links, so may be relevant.

The air quality sensor recognizes carbon monoxide but is not designed as a safety feature to protect you from carbon monoxide. The air quality sensor is designed to limit your exposure to gasoline exhaust fumes and or diesel exhaust fumes that could enter into the passenger compartment from nearby vehicles for a limited amount of time.

The HVAC control module monitors the outside air quality while in AUTO mode via the air quality sensor assembly. You can activate the air quality sensor on your vehicle, if equipped, by pressing the Auto button on the HVAC module. The air quality sensor is a 5-wire sensor, 1 ignition voltage circuit, 2 ground circuit, and 2 control circuits. There are 2 sensors inside the air quality sensor assembly. A hydro carbon (HC) sensor that is used for detecting gasoline exhaust fumes and a Nox sensor that is used for detecting nitrous oxide, diesel exhaust fumes. The HVAC control module outputs a 5-volt signal on the HC and Nox control circuits and operates in a range between 0-5 volts.

The HVAC module updates the air quality sensor calibration approximately 4 times per second. The air quality sensor input to the HVAC module is compared to the previously updated air quality sensor calibration. If the voltage difference is great enough the recirc actuator will be controlled to recirc, outside air, or partial recirc outside air position based on the air quality sensor input.

When air quality around the front of the vehicle is poor, the air quality sensor system operates as follows:

Poor air quality with high HC levels will cause the HC control circuit to read a lower voltage.
Poor air quality with high Nox levels will cause the Nox control circuit to read a higher voltage.
The recirculation actuator is forced to the recirculation position.
The recirc symbol on the HVAC module will illuminate.
The recirc icon will not illuminate if the recirc actuator is already in the auto recirc position.

When air quality around the front of the vehicle is normal, the air quality sensor system operates as follows:

Normal air quality levels the HC control circuit will recognize very little voltage change from the previously updated calibration.
Normal air quality levels the Nox control circuit will recognize very little voltage change from the previously updated calibration.
The recirculation actuator position is not controlled by the air quality sensor system.

The following are conditions that will prevent the air quality sensor from controlling the recirc actuator:

A/C OFF conditions (compressor OFF) prevents air quality sensor from changing the current position of the actuator)
Coolant temperature below 37°C (99°F) during warm up will prevent the blower from operating and air quality sensor from controlling the recirc position.
Outside air temperature less than 5°C (41°F) (prevents air quality sensor from changing the current position of the actuator)
Outside air temperature less than 15°C (59°F) and A/C high side pressure less than 196 kpa (28 psi) (prevents air quality sensor from changing the current position of the actuator)
28 seconds sensor warm-up time after ignition is turned ON (prevents air quality sensor from changing the current position of the actuator)
Evaporator temperature sensor failure
Customer manually selects defrost or defog (forces actuator to outside air position)
Customer manually selects recirc or outside air position
Coolant temperature above 112°C (V8), 119°C (HFV6) the actuator is forced to the recirc position. When coolant temperature drops below 109°C (V8), 116°C (HFV6) forced recirc is canceled.
A/C pressure above 2 500 kpa (362 psi) the actuator is forces to the recirc position. When A/C pressure drops below 2 000 kpa (290 psi) forced recirc is canceled.

The following describes the recirc actuator function after the air quality sensor has canceled the command to recirc

Full recirc position will be held for 2 minutes when vehicle speed is less than 10 km/h (6.3 mph) after 2 minutes expires the recirc door will move to full outside air position.
Partial recirc, outside air position will be held for 2 minutes when vehicle speed is above 20 km/h (12.5 mph) after 2 minutes expires the recirc door will move to full outside air position.
Full outside air position if vehicle speed is greater than 60 km/h (37 mph) and blower is not on high speed.

The Air Quality system will have three Sensitivity levels, Low, Medium, and High. Medium is the default setting in new vehicles.

The Sensitivity Level can be changed through SPS programming. If exhaust fumes are noticed in the vehicle a more sensitive level should be selected.

The driver information center (DIC) will display the SERVICE A/C SYSTEM message if the HVAC system has detected a short to ground in the air quality sensor circuit. An open circuit or a short to voltage in the air quality sensor circuit will be diagnosed through symptom based diagnostics because both of these conditions will not turn on the SERVICE A/C SYSTEM message or set a trouble code.