Hey, Why Is My Car's AC Blowing Hot Air?

The proof image shows the location of the AC control head before disassembly. The control head is part of the HVAC system that the driver uses to adjust the air conditioning settings, such as temperature and fan speed.

A customer brought their vehicle, reporting an issue with the air conditioner's fan speed. They explained that the fan speed would randomly drop and increase, despite being set to a certain speed. They suspected the blower motor and resistor might be broken, as they had experienced a similar issue with a previous vehicle.

During a test drive, the technician confirmed that the fan control was malfunctioning. However, upon the vehicle health inspection, the technician found nothing to explain the customer's concerns. To continue diagnosing the issue, the customer authorized additional tests.

The technician then accessed the blower motor resistor and used a digital voltmeter to check if the signal from the control head was sending the correct information to the resistor module.

The technician discovered that the fan control coming from the control head was erratic and not sending the appropriate signal to the blower motor resistor module. Based on this, they concluded that the control head was the issue and needed to be replaced.

The proof image shows the location of the evaporator temperature sensor. In this case, this part was broken and caused warm air to come out of the AC.

A customer dropped off their vehicle, reporting that their air conditioning appeared to work well during the spring season. However, now that the temperature has increased, the air conditioning system only blows warm air.

During the test drive, the technician noticed that the AC was warm and the check engine light was on. However, the technician found nothing related to the customer's concern during the vehicle health inspection. To investigate further, a vehicle code scan was performed, and it revealed an AC evaporator temperature sensor circuit low (P0537 code).

After receiving authorization for specialized procedures, the technician used a vehicle-specific scanner to check the climate control module's live data. The technician found that the evaporator temperature sensor was reading -40F, indicating a malfunction.

The -40F reading is an extreme deviation from the usual 32F to 40F degrees, indicating a sensor malfunction.

The technician then accessed the temperature sender in the airbox and verified that the wiring was intact, so it wasn't a wiring issue. Unfortunately, this confirmed that the evaporative temperature sensor was failing, resulting in the AC system's malfunction.

The image shows a faulty body control module that had lost the drivers to engage the AC compressor.

A customer dropped off their vehicle, reporting that their air conditioning wasn't cooling well. The AC worked during the early morning hours but not after that. The customer did not notice any leaks or strange odors coming from the vehicle, and they checked the compressor fuse, which appeared to be okay.

During the test drive, the technician confirmed that the AC did not cool. Then, during the vehicle health inspection, the technician found that the air conditioning compressor did not engage even when the AC was turned on.

After authorization for additional diagnostic procedures, the technician back-probed the AC compressor and found that no voltage went to the AC compressor clutch. To test further, the technician jumped power to the AC compressor clutch, the compressor engaged, and the air conditioner started to cool. The technician then checked the fuses and confirmed that they were okay.

The technician used a vehicle-specific scanner to check if the body control module was commanding the AC clutch to engage. Based on live data, the module commanded the AC compressor to activate.

However, the technician then used the wiring schematic and found that one wire from a control module connector was the command wire going to the relay. When commanding the AC, no voltage came out on the said wire. The technician then jumpered power to that wire, and the compressor came on. So, the body control module was faulty.

The proof image shows the location of the faulty AC compressor relay that was stuck, causing the AC to run constantly and draining the battery.

A customer brought their vehicle to the auto shop complaining they had to replace its battery thrice a month. Plus, if they left their car parked for less than an hour, it needed to be jump-started; they wondered if the alternator was failing.

The battery was dead, and the technician had to jump-start the vehicle, but no other issues related to the customer's concern were noted. During a vehicle health inspection, the technician checked the battery, charging, and starting system, and everything seemed to work fine. However, additional tests were authorized.

First, the technician used an amp meter to test the vehicle's current drawn with the ignition off. A spark indicated a significant load as the technician was about to hook up the amp meter in series with the negative battery cable. They then hooked the battery cable back up and found a 25-amp draw on the battery with the engine off and the key out.

The technician unhooked the battery cable again and heard the air conditioning compressor clutch click. Using the vehicle service manual, the technician located the AC compressor clutch relay and removed it from the relay box. They found that the key off draw was less than 50 milliamps, which is within specifications.

The technician then plugged in the compressor clutch relay again, but with no accessories on, they found the relay stuck. Therefore, the technician determined that the relay was causing a parasitic draw on the battery and needed to be replaced.

The proof image shows the faulty ambient air temperature sensor.

A customer brought in their vehicle, mentioning their air conditioning system wasn't working well.

The technician test-drove the car, found that the air conditioning wasn't working correctly, and noted that the check engine light was on. The technician then did a vehicle health inspection and found nothing that could be causing the problem. However, when they scanned the vehicle's system for error codes, they found a p0073 code, indicating an issue with the Ambient Air Temperature Sensor circuit.

Since the customer authorized additional diagnostics, the technician used an air conditioning machine to check the system's pressure. They found that the pressure was expected, but the temperature inside the car was still high, reaching around 82° F.

To further investigate, the technician removed the freon from the air conditioning system and found that the amount recovered was optimal for the vehicle. They then recharged the system and checked the readings with a dedicated scanner.

The technician found that the Ambient Air Temperature was reading -40 °F, so the system received a signal that the outside temperature was much colder than it was. The technician then examined the sensor and the wiring but didn't see anything faulty. They verified that the wiring between the sensor and the ECM (Engine Control Module) was okay, concluding that the Ambient Air Temperature Sensor was failing.

The Ambient Air Temperature Sensor is an essential part of the air conditioning system because it provides information to the ECM about the outside temperature, which helps the system regulate the temperature inside the car.

If the sensor is not working correctly, it can cause the air conditioning system to malfunction and not cool properly. Therefore, the technician determined that replacing the sensor was the best solution to the air conditioning issue.