A stuck expansion valve (TXV) causes erratic cooling. Stuck open floods the evaporator and pressure swings wildly. Stuck shut starves the evaporator and the high side pressure spikes.
The remote bulb that senses evaporator outlet temperature has leaked its own charge. Valve defaults open or closed depending on design. Pressures behave erratically.
Particles from a failing compressor lodge in the valve seat. Valve cannot close or fully open. Often paired with a clogged inlet screen at the TXV.
Saturated drier sends water through. It freezes at the TXV and blocks flow. Symptoms come and go as ice forms and thaws.
Internal spring broken or diaphragm torn. The valve cannot modulate. Replace as a unit.
Stop-leak products gum up the valve internals. One of the reasons most shops refuse to service systems with sealant.
Severe overcharge or undercharge can mimic a stuck TXV. Always verify charge before condemning the valve.
After a previous repair, the bulb was not insulated or was attached incorrectly to the evaporator outlet. Reading is wrong, valve hunts.
| Likely Cause | Typical Cost | DIY Difficulty | Severity | Likelihood |
|---|---|---|---|---|
| TXV Sensing Bulb Lost Charge | $200-$500 TXV + 2-4 hrs | Hard | Medium | 55% |
| Debris Blocking Valve | $300-$700 TXV + flush | Hard | High | 50% |
| Moisture Freezing at Valve | $200-$400 + drier | Hard | Medium | 35% |
| TXV Mechanical Failure | $200-$500 + 2-4 hrs | Hard | Medium | 25% |
| Stop-Leak Residue | $400-$800 cleanup + parts | Hard | High | 20% |
| Wrong Refrigerant Charge | $80-$200 charge correction | Hard | Low | 15% |
| TXV Bulb Improperly Installed | $30-$100 reinstall | Moderate | Low | 10% |
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If your scanner is showing one of these, that is your starting point. Tap any code for full causes and repair costs.
Thermal expansion valve. It meters refrigerant into the evaporator based on outlet temperature. More sophisticated than an orifice tube. Found on most modern systems.
Pressures behave erratically on the gauges. Cooling cycles cold-warm-cold. Frost may form on one side of the valve while the other side is warm. A shop with proper gauges can diagnose in 15 minutes.
On most cars, at the firewall where the AC lines enter the cabin. Behind a metal cover, usually right next to the evaporator inlet.
$300-$700 at a shop including part, drier, evac, and recharge. The part itself is $30-$100.
Sometimes for very small debris. But the part is cheap relative to the labor and you have already paid for the labor. Replace it while you are in there.
10-15 years on a healthy system. They usually outlast the rest of the AC system. When one fails, often something else (drier or compressor) caused it.
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