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Troubleshooting an Automatic Climate Control System

Climate controls

Automatic climate control systems are very convenient, but can be costly to repair if something goes wrong. Diagnosing these systems can be time consuming and frustrating. If you understand how they work and what is most likely to go wrong, however, things can be much simpler.

An automatic climate control system works by automatically opening and closing vents, engaging or shutting off heat or air conditioning, and controlling the blower fan(s). Temperature sensors in the cabin tell the climate system's computer when to adjust things. For the passengers in the cabin, all that's required is that a preferred temperature be set and the rest is automated.

Components of the Automatic Climate Control System

There are several components to the ACCS. Much of the climate control is the same as it would be for a manual system that requires driver or passenger input to change temperature outputs or airflow. Physical controls for the vents inside the air ducts that control airflow to various external vents are powered by actuators that move them electronically.

Outside of those actuators, most of the differences are in the control unit and the various sensors that give input to the CU. The control unit for ACCS is usually located either directly behind the climate controls on the dashboard or in the cabin's fuse box.

Sensors will include an ambient air (external) temperature sensor and an internal (in-cabin) temperature sensor. Often there are multiples of these, especially in luxury vehicles. Specialized sensors such as those which read body temperature via infrared, "sunload" sensors that take the temperature of the sunlight entering the cabin through the windows, and similar are uncommon, but may be found in some higher-end vehicles.

For each "climate zone" in a vehicle, separate sensors will be operating inside each of those zones. So a "tri-zone" crossover or minivan, for example, will have temperature sensors on the driver's side, the passenger's side, and in the rear cabin of the vehicle.

Control units can also be simple or complex. Some are simple one-unit module that simply compares inputs from sensors with a "grid" of options and activates the appropriate responses in a way similar to electronic ignition timing or transmission shifting. Others are much more complex with multiple control modules controlling various portions of the climate controls and communicating via a sophisticated network of wiring.

Troubleshooting the Automatic Climate Control System

To start with, troubleshooting the ACCS will be far easier if you have a scan tool specific for in-vehicle HVAC. These are specialized tools and they are not commonly found in the do-it-yourselfer's garage. Many repair shops don't even have one. Some auto parts stores may have one to rent or will scan the codes for you free of charge in the parking lot.

Fault codes are not always universal or standard. Depending on the vehicle and the sophistication of its ACCS, these codes can be very useful or only generally point in the right direction. The rest is up to fault testing and circuit checks. Replacing parts willy nilly is usually not an option as a BUS, module, or sensor for an ACCS can be very expensive.

Common ACCS Problems

Outside of problems that can be found in any climate control system, a few specific issues can be had with an ACCS.

An ACCS that doesn't operate at all (does nothing no matter the temp setting) likely has a main module malfunction, a blown fuse, or system short. You may also have to reset the control unit. Check your owner's manual.

When temperatures do not match the setting on the controls, the most likely issue is a faulty sensor or sensors. This assumes that blend doors and actuators are working properly, of course.

Speaking of blend doors, if air is flowing out of some ductwork but not others, that is almost always the cause. This is likely a bad actuator or a stuck door (that will quickly lead to a bad actuator if not released).

If the system is working, but does not get aggressive enough about adding heat or cold when the temperature outside or coming through the windows warrants it, the problem is likely a bad ambient air sensor or window "sunlight" sensor. The former can be quickly checked by locating the temperature readout for the ambient (outside) air and checking its accuracy. This readout is usually in the instrument cluster or on the radio/infotainment screen.