A thermostatic expansion valve (TXV) fails in one of two opposite ways, and they produce opposite symptoms. A valve stuck closed starves the evaporator: high superheat, low suction pressure, weak cooling. A valve stuck open floods it: low or near-zero superheat, and a real risk of liquid refrigerant reaching the compressor. Telling these apart, rather than treating “bad TXV” as one symptom set, is the actual diagnosis.
How a TXV works, briefly
The TXV meters liquid refrigerant into the evaporator based on superheat, the temperature difference between the refrigerant leaving the evaporator and its saturation temperature at that pressure. A sensing bulb on the suction line feeds this reading back to the valve, which opens or closes to keep superheat in a target range, typically 8-12°F on a properly charged residential system, though the exact target varies by equipment and manufacturer spec.
Failure mode 1: stuck closed (underfeeding / starved evaporator)
- High superheat, well above the normal range, since too little refrigerant is reaching the coil to absorb heat efficiently.
- Low suction pressure, because the restricted flow starves the low side of the system.
- Reduced cooling capacity: warm air at the registers, longer run times, and a system that can’t hold setpoint on a hot day.
- Frost pattern: if frost forms at all, it’s typically limited to the evaporator inlet, not spread across the whole coil, since only the first section is getting enough refrigerant to drop below freezing.
Failure mode 2: stuck open (overfeeding / flooding)
- Low or near-zero superheat, since excess liquid refrigerant is reaching the evaporator outlet without fully vaporizing.
- Liquid floodback risk: unvaporized liquid reaching the compressor can wash oil off the cylinder walls and, in severe cases, cause slugging, a leading cause of compressor failure from a TXV problem specifically.
- Frost pattern: the evaporator coil, and sometimes the suction line itself, can frost heavily across its full length rather than just the inlet.
Why subcooling matters for telling this apart from a charge problem
Subcooling is measured at the condenser outlet, upstream of the TXV, and reflects how well the condenser is doing its job independent of how the valve is metering refrigerant into the evaporator. A charge problem (undercharge or overcharge) typically pulls both superheat and subcooling out of range together, in a consistent direction. A TXV-specific problem more often shows abnormal superheat while subcooling stays closer to its normal range, since the condenser side of the system isn’t where the fault is. Measuring both, not just one, is what actually separates “bad TXV” from “wrong charge” rather than guessing between them.
| Reading | Stuck closed (starved) | Stuck open (flooding) |
|---|---|---|
| Superheat | High | Low / near-zero |
| Suction pressure | Low | Elevated or erratic |
| Subcooling | Near normal | Near normal |
| Frost pattern | Inlet only, if any | Full coil, possibly the suction line |
| Main risk | Poor cooling, compressor overheating | Liquid slugging, compressor damage |
Safety box: confirming a TXV diagnosis requires refrigerant gauges and EPA Section 608 certification to access the system. Don’t attempt to add or remove refrigerant to “test” a suspected TXV problem; a wrong charge on top of a valve fault makes the diagnosis harder, not easier.
Common TXV brands and how they’re rated
Sporlan (part of Parker Hannifin), Danfoss, and Emerson (Alco/Emerson Climate) make most of the TXVs found in residential and light-commercial systems. Valve capacity ratings, the tonnage a given valve size is designed to meter, are standardized under AHRI Standard 750, which is why a replacement valve rated for the same tonnage as the original generally performs comparably regardless of brand, as long as the connection type and refrigerant match.
Measuring superheat correctly matters as much as the reading itself
A superheat reading taken at the wrong point, or without letting the system run long enough to stabilize, can look abnormal on a perfectly good valve. Take the reading at the outlet of the evaporator (or the suction line near the TXV’s sensing bulb), after the system has run for at least 10-15 minutes at steady load. A reading taken during startup, or right after a defrost cycle on a heat pump, isn’t representative of normal operation.
Both failure patterns can also show up as a compressor that short-cycles or trips on internal overload protection, since either starving the compressor of return gas (stuck closed) or flooding it with liquid (stuck open) puts it under abnormal stress. If the compressor is cycling on its own overload protector rather than the thermostat’s call, that’s worth mentioning to whoever diagnoses the system, since it narrows down how far the fault has already progressed.
Frequently Asked Questions
Can a bad TXV cause high head pressure? A stuck-open TXV can contribute to elevated head pressure indirectly, since excess refrigerant reaching the evaporator can back up through the system, but head pressure abnormalities are more commonly a condenser-side issue (dirty coil, failing fan, overcharge) than a TXV symptom specifically.
Is a stuck TXV the same as a clogged filter drier? No, though they can produce similar starved-evaporator symptoms (high superheat, low suction pressure). A filter drier restriction is a blockage anywhere refrigerant flows through it; a TXV problem is specifically a metering failure at the valve itself. A tech distinguishes them by checking pressure drop across the drier versus across the valve.
How much does TXV replacement cost? Parts alone typically run $50-200 depending on the valve; total cost installed, including recovering and recharging refrigerant, is commonly $300-800 at most residential rates, more on larger commercial systems.
Can I replace a TXV myself? No. It requires recovering refrigerant, replacing the valve, evacuating the system, and recharging to spec, all of which need EPA 608 certification and the right equipment. This is a licensed-technician job.
Related: what an AC capacitor does and what a contactor does for the electrical side of a no-cooling diagnosis, if the TXV checks out normal.
