Air Conditioner Troubleshooting Guide & Diagnosis

Last updated: October 27, 2026

In this guide (7 sections)

1. Common Mercedes A/C Problems
2. How the A/C System Works
3. Troubleshooting the A/C System, Step by Step
4. Common A/C Problems and What to Actually Do
5. A/C System Gauge Readings
6. Related Reading
7. References

Is your air conditioner A/C not working or blowing hot air? The air conditioning system can get very complicated in cars, but that doesn’t mean that you can’t perform some troubleshooting yourself. Maybe even charge the A/C system if needed. This article provides an overview of the A/C system, common problems, and basic troubleshooting you can perform yourself. While the pictures in this article are of Mercedes-Benz cars, this guide is written to apply to all makes and models.

Common A/C Problems
How the A/C System Works
Troubleshooting the A/C
Problems and Solutions
Gauge Readings
Related Reading

Common Mercedes A/C Problems

• A/C blowing warm or hot air
• Low A/C system pressure or low refrigerant charge
• Refrigerant leaks
• Compressor clutch or compressor control problems
• Liquid lock of the compressor
• Compressor cycles on and off rapidly

How the A/C System Works

One of the key components of the A/C system is the compressor. The compressor’s purpose is to add pressure to the system by compressing the refrigerant. The operation of the A/C in your car is based on the First Law of Thermodynamics if you want to learn the physics behind it. From the compressor, the fluid goes to the condenser. By flowing through the condenser, the refrigerant loses heat, which converts into a warm liquid from hot gas.

Next in the system is the receiver-drier, whose primary purpose is to remove moisture from the A/C refrigerant. After that, the refrigerant goes through the expansion valve. The pressure drops in the expansion valve, allowing the refrigerant to expand. The expansion of the refrigerant causes the temperature of the refrigerant to drop. This is the opposite process that the compressor performs. Putting pressure into a fluid will increase its temperature. Removing pressure from a fluid causes it to lose temperature. So in the expansion unit, the refrigerant temperatures drop below the outside temperature. From here, the refrigerant goes to the evaporator.

The evaporator works similarly to the radiator, except in this case, instead of hot coolant, there is cold refrigerant flowing through the evaporator. The low-temperature refrigerant passes through the evaporator, which causes the evaporator to get cold. The air then blows through the evaporator, which makes it get cold.
This is the essential operation of the A/C system, so let’s move on to some troubleshooting tips.

Troubleshooting the A/C System, Step by Step

1. Gather the right tools, including safety glasses and gloves, because liquid refrigerant escaping a fitting will frostbite skin and eyes. Check the underhood A/C label before adding refrigerant. Use only the refrigerant and charge specification listed for your vehicle. You will need a recharge kit, which can be purchased at a local auto parts store or online on Amazon. It has a gauge you will first use to test your A/C system.
2. Turn the A/C ON. If you have an Economy button, ensure it is turned off. Ensure the fan is high, the temperature selector is turned to COLD, and the A/C and recirculate light indicators are on.
   • Those are all on the climate control panel.
3. Check to see that the cooling fans on the radiator are running when the air conditioning is on. Most cars are designed to have the radiator cooling fan kick in when the A/C is on.
4. Check that the compressor has engaged. Identify the location of your compressor. Determine which compressor design the vehicle uses before testing it. On a magnetic clutch compressor, verify clutch engagement. On a variable displacement or electric compressor, use fault codes, live data, pressure readings, and supported active tests to verify compressor command and operation. With the engine running and the A/C set as above, look at the pulley and check that the clutch is engaging. The pulley spins any time the engine is running. The center part of the A/C pulley only engages when the A/C is on. If the clutch engages and disengages rapidly, the refrigerant may be low.
5. Locate the Low-Pressure A/C recharge port. Attach the hose from the A/C Recharge Kit and check the pressure. Ensure the car is running and the A/C is on while you get a reading.
6. If the system is low, start recharging. Follow the recharge kit manufacturer’s instructions. Pay attention to the direction in which you need to hold the can. Some kits ask that you keep the can upright, while others ask to keep it upside down. Follow the A/C recharge kit recommendation to avoid putting air into the system.
7. If the reading moves toward the normal range, continue until the system is charged. Understand what a can top-up is and is not: it confirms the diagnosis and gets you cold air, but a system that was low got low because it leaks, and the only accurate charge is by weight after the old refrigerant has been recovered and the system pulled down under vacuum. Refrigerant has to be recovered rather than vented when the system is opened.
   • If you are charging the system, but the gauge is not moving, you may have a large leak. You will need to charge the system with dye to locate the leak.
   • If the system works but only for a short time, you have a leak that needs to be located and repaired.
   • You have a minor leak if the system works for a few months. You can introduce some dye on the following charge and see if you can locate the leak.
   • As you charge the system, you may notice the A/C starts to work. The A/C system is designed to disengage the A/C if the pressure in the lines is too low. This happens to protect the system. The compressor could be disengaged because the system does not have enough refrigerant pressure to trip the low-pressure cutoff switch that cycles the compressor.
   • Be careful not to overcharge the system, either. Overcharging can damage the A/C system and reduce cooling performance. You should charge the system while the car is running.
   • If you have low pressure, then check for leaks. Leaks are the most common problem associated with the air conditioning system. Charge the system via the low-pressure port with a Leak Detection Kit like this so you can find the leak.
     • Hopefully, your system was low, and it is working again. If not, continue with these troubleshooting steps.
8. Check the fuses. Look under the hood or under the dash to find the fuses. Find the A/C fuse and make sure that it is not blown.
   
9. Next, check the clutch itself. The clutch may be bad. Is there voltage going to the clutch? A cycling switch may be bad, a fuse may be blown, and the system may not have enough refrigerant pressure to trip the low-pressure cutoff switch that cycles the compressor. Make sure you know what type of compressor you have.
10. Check the relays.
    
11. Check the operation of the climate control and the A/C button.
12. Check the cabin air filter to ensure that it is not clogged.
13. Check the blower fan. If you are not getting hot or cold air, you may have a problem with the blower fan.

NOTE:

• The dye check will not work if the system is too low and the compressor is not cycling.
• There is also a high-pressure service port. A high-pressure gauge can be handy in further troubleshooting the system and is what the professionals use. Your AC recharge kit will not work; it doesn’t fit on the high-pressure port.
• Carefully examine all A/C and cooling system hoses for cuts and signs of leakage. Leakage from A/C system hoses is often indicated by an accumulation of dirt and oil, particularly at connections and fittings.
• Ensure the condenser (in front of the radiator) is free of obstructions, such as leaves or insects. This could reduce airflow, resulting in reduced A/C performance. You can rinse the condenser clean with a garden hose.
• If the basic A/C checks look normal, scan the climate control system before replacing parts. Check fault codes, refrigerant pressure sensor data, temperature sensor readings, and supported compressor commands.

Diagnosing problems with an air conditioning system can be very complicated. This section will cover some common problems associated with the A/C system. Always wear safety glasses and gloves. Keep all loose-fitting clothing and body parts away from moving parts. A professional mechanic or technician should be consulted for any problems not covered below.

The tables below are organised by what your gauges show you, because the pressure pattern is what narrows the cause. They apply to any make, and where a reading is specific to Mercedes it is called out.

Common A/C Problems and What to Actually Do

Open the group that matches what your gauges or your symptoms are telling you.

Low discharge pressure, low on the high side

The compressor is turning but it is not building pressure, or there is not enough refrigerant in the system for it to work with.

Likely cause	How you confirm it	What to do
Leak in the system	Oily dirt at a fitting, a line, the condenser face or the compressor shaft seal. Add dye and run it, or use an electronic leak detector	Repair the leak, then evacuate and recharge by weight. Topping up a leaking system is money poured out through the hole
Low refrigerant charge	Both sides read low and the clutch cycles rapidly or will not engage	Find the leak first, then recover, evacuate and charge to the weight on the underhood label
Blocked receiver drier or filter	A noticeable temperature drop or frost across the drier	Replace the drier, and evacuate under vacuum before recharging
Faulty expansion valve or orifice	Low side very low, high side also low, evaporator barely cools	Replace the expansion valve. Replace the drier at the same time
Worn or internally damaged compressor	Pressures stay close together with the clutch engaged, and the compressor may be noisy	Replace the compressor. Automotive compressors are not rebuilt in place, so reed valves and head gaskets are not a repair here

High discharge pressure, high on the high side

The system cannot get rid of the heat it is collecting. This is nearly always airflow or overcharge, and it is the condition most likely to kill a compressor if you keep driving.

Likely cause	How you confirm it	What to do
Air or moisture in the system	High side reads high and erratic, cooling is poor, the system was opened without being evacuated	Recover the charge, replace the receiver drier, then pull a deep vacuum for 30 to 45 minutes to boil the moisture out. Only then recharge by weight. Adding more refrigerant on top of air does nothing at all
Blocked or dirty condenser face	Leaves, bugs and road dirt packed into the fins, visible through the grille	Rinse the condenser from the engine side out with a garden hose, gently. Straighten bent fins
Auxiliary cooling fan not running	The fan should spin up when the A/C is on. If it does not, high side climbs as soon as the car stops moving	Check the fan fuse and relay, then the fan control module and the fan motor itself. On many Mercedes the fan runs on a control module that fails more often than the motor
Overcharged system	Both sides read high, vent temperature is poor despite a full charge	Recover the excess and charge by weight, not by gauge feel. Overcharging is the most common mistake made with a single gauge can
Worn belt or failed tensioner	Belt squeal, glazed belt, compressor pulley slipping under load	Replace the belt and the tensioner. A modern serpentine belt is not adjusted by tightening it

Low suction pressure, low on the low side

The evaporator is being starved. Either not enough refrigerant is reaching it, or something is restricting the flow.

Likely cause	How you confirm it	What to do
Refrigerant shortage	Both sides low, clutch cycling quickly	Leak test, repair, then evacuate and charge by weight
Restriction in the expansion valve or screen	Frost or a cold spot right at the valve, with warm lines downstream	Replace the expansion valve and the receiver drier together
Moisture in the system freezing at the valve	Cooling works for a few minutes then quits, and comes back after the car sits	Recover, replace the drier, pull a deep vacuum, recharge. Moisture is removed by vacuum, not by adding refrigerant
Kinked or crushed line	Visible damage, or a sharp temperature change along one hose	Replace the line
Internally worn compressor	Suction pressure does not pull down even with a correct charge	Replace the compressor, the drier and the expansion valve

High suction pressure, high on the low side

The expansion valve is letting too much refrigerant through, or the compressor is not pulling the low side down.

Likely cause	How you confirm it	What to do
Expansion valve stuck open	Low side high, high side normal or low, vents only mildly cool	Replace the expansion valve
Overcharged system	Both sides high	Recover and charge by weight to the underhood label
Compressor not sealing internally	Both sides drift toward each other with the clutch engaged	Replace the compressor along with the drier and expansion valve

Compressor is not running at all

The clutch never pulls in, or on a variable displacement or electric compressor it is never commanded. This is the group where a scan tool saves the most money.

Likely cause	How you confirm it	What to do
Low refrigerant tripping the pressure switch	Pressure below roughly 2 bar. The car deliberately refuses to engage the clutch to protect the compressor	Leak test, repair, then evacuate and recharge. The clutch will engage once pressure is restored
Blown fuse or failed relay	No 12 V at the clutch connector with the A/C commanded on	Replace the fuse or relay, then find out why it failed
Open clutch coil or broken clutch wire	Measure the clutch coil resistance. An open coil reads infinite, a healthy one reads a few ohms	Replace the clutch coil or repair the wiring
Evaporator temperature sensor reading falsely cold	The climate module cuts the compressor to stop the evaporator icing. Live data shows a near freezing evaporator while the vents blow warm	Replace the evaporator temperature sensor. This one wastes a lot of refrigerant money because the system is actually fine
Climate control module not commanding the compressor	No fault at the clutch, no refrigerant fault, and live data shows no compressor request	Read codes across all modules. The request can be blocked by an engine side fault or a dead module
Seized compressor or snapped belt	The pulley does not turn, or the belt is gone	Replace the belt, or the compressor along with the drier and expansion valve

Vents blow cool but never cold, or the evaporator ices up

Here the refrigerant side is usually healthy and the problem is airflow, air distribution, or a sensor.

Likely cause	How you confirm it	What to do
Blocked cabin air filter	Weak airflow at every vent on every speed	Replace the cabin filter. This is the cheapest fix on the page and it is the cause more often than people expect
Evaporator iced over	Airflow drops away after ten or twenty minutes of use, then recovers after the car sits	Let it thaw, then find out why. A faulty evaporator temperature sensor or a blocked drain are the usual reasons
Blocked evaporator drain	Water in the footwell, a damp smell, fogged glass	Clear the drain
Stuck monovalve or blend flap	One side blows cold and the other warm, or the temperature setting makes no difference	Compare the left and right heater core temperatures in live data. A large split points at one stuck monovalve
Hot air leaking into the cabin	Cooling is poor only at low speed or in traffic	Check the door and window seals and the blower housing seals
Failed blower motor or regulator	Airflow is weak or fixed at one speed	Replace the blower motor or its regulator

A/C System Gauge Readings

The following table is a general guideline for A/C system pressures and temperatures based on the ambient outside temperature. Remember that these are guidelines, and your actual temperatures and pressures will vary depending on the humidity in the air and the condition of your system.

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Related Reading

References

1. Clark’s Garage HVAC shop manual
2. Troubleshooting with gauges, aircondition.com
3. A/C diagnosis, aa1car
4. Automatic climate control, aa1car
5. How mobile A/C works, MOVACS