Your car’s AC low side pressure is too high: What’s the cause?

Learn what to do if your car’s AC low side pressure is too high

If your car’s AC low side pressure is too high, there can be many causes, from a refrigerant overcharge to something as simple as a clogged cabin air filter. I’ll go over each possibility and explain how to test and fix the problem

Manifold gauge set with hoses

Possibility #1 AC low side pressure is too high because the system is overcharged

If you’ve recently added refrigerant to the system and are now seeing high low side pressure and warm air coming out of the vents, you may have added too much refrigerant. Evacuate the system and recharge with the exact amount of refrigerant based on the car maker’s specifications.

One way to check for an overcharged condition is to spray water over the condenser fins while the system is running. If low side pressure drops significantly, that’s an indication of an overcharged system.

Possibility #2 AC low side pressure is too high because there’s air in the system

An AC system is a closed system. If you’re low on refrigerant, it’s because you have a leak. When refrigerant leaks out, air leaks in, and air, no matter how much you compress it, will not cool.

One way to check for air in the system is to watch the high-pressure reading. Run the system for about 5 minutes and shut it off. If the high side pressure drops from around 150-200 to around 28 psi quickly and then falls gradually after that, you’ve got air in the system.

Possibility #3 AC low side pressure is too high because the condenser has reduced airflow

Clogged condenser fins or an inoperative condenser fan can cause low-side pressure to be too high. Clean the condenser fins and ensure the condenser fan is running at the proper speed.

Possibility #4 AC low side pressure is too high because the cabin air filter is clogged

If cabin air can’t pass across the evaporator coil, the refrigerant can’t change from a liquid to a gas, resulting in high low side pressure and warm air coming from your vents. Clogged evaporator core fins can cause the same problem

High low side pressure due to a faulty expansion valve

If the expansion valve is stuck open or not metering the refrigerant flow correctly, it can allow too much refrigerant into the evaporator, raising the low-side pressure

Blockage in the system

Blockage or restriction: A blockage or restriction in the refrigerant lines or components on the low-pressure side can impede the flow of refrigerant, increasing the pressure.

©, 2020 Rick Muscoplat