Every car AC system has a point where high-pressure liquid refrigerant is let through a small opening into the evaporator inside the dash. That drop in pressure is what makes it cold. Depending on the vehicle, that job is done by one of two parts.
Expansion valve or orifice tube?
- Thermostatic expansion valve (TXV). A small valve, usually at the firewall where the AC lines enter the dash, that adjusts the flow based on evaporator temperature. Systems with a TXV have a receiver drier on the high side.
- Orifice tube. A fixed tube with a fine mesh screen, sitting inside one of the AC lines. It cannot adjust, so these systems use an accumulator on the low side and cycle the compressor on and off to control temperature.
Which one your vehicle has depends on the make and model. Many Ford and GM vehicles used orifice tubes; most Asian and European makes use expansion valves.
Signs of a stuck or clogged valve
- Air that is cool but never properly cold
- AC that starts cold and then fades to warm on a long drive
- Frost or ice forming on the AC lines or at the firewall
- Rapid compressor cycling, clicking on and off
- Gauge readings that do not make sense for the charge level: a very low low-side reading points to a restriction, a high low-side reading can mean the valve is stuck open
Why they clog
The opening is small on purpose, so it is the first place debris stops. Metal from a failing compressor, desiccant from a broken drier, and stop-leak sealer from store-bought refrigerant cans all end up here. Moisture in the system can also freeze at the valve and block it intermittently.
Replacing it
The expansion valve or orifice tube is routinely replaced whenever a compressor is replaced, because it has almost certainly caught debris. On its own, it is a smaller repair, although access varies a lot: some valves are right at the firewall, and some need dash or console parts removed. We tell you which kind your vehicle has before quoting.
The system has to be recovered, opened, and then put through a full vacuum and recharge afterward, which is part of any job that opens the refrigerant circuit.