Frost or ice on AC lines under the hood means the low-side pressure is dropping below freezing. Almost always a low charge, a blocked expansion device, or a moisture-saturated drier.
When pressure on the low side drops below about 20 PSI, line temperature drops below freezing and atmospheric moisture condenses and freezes on the outside of the lines. Recharge with dye to confirm and find the leak.
Debris partly blocks the valve. Refrigerant flashes too early, line gets very cold, ice forms. Pressures swing wildly when the valve hunts.
Moisture in the system freezes at the expansion device. Ice slowly grows on the lines. Symptom intensifies over a long drive.
A failed cutoff lets the compressor run even when pressure is unsafe. Low side drops too far, line freezes.
Some systems have a small filter screen at the expansion valve. Clogs from compressor metal debris cause the same restriction symptoms as a bad valve.
Sensor that protects against icing has failed. Compressor never cycles off, evaporator overcools, lines freeze.
Clutch coil shorted or stuck-engaged compressor keeps pumping below cycle pressure. Long runs lead to severe line icing.
| Likely Cause | Typical Cost | DIY Difficulty | Severity | Likelihood |
|---|---|---|---|---|
| Low Refrigerant Charge | $50-$200 recharge with dye | Easy | Low | 65% |
| Clogged Expansion Valve | $200-$500 + 2-4 hrs | Hard | Medium | 45% |
| Saturated Receiver-Drier | $150-$400 evac, drier, recharge | Hard | Medium | 35% |
| Bad Pressure Switch | $25-$75 + 0.5 hr | Easy | Low | 25% |
| Restricted Filter or Screen | $100-$300 part + labor | Hard | Medium | 20% |
| Failed Evaporator Thermistor | $40-$120 + 1-2 hrs | Moderate | Low | 15% |
| Stuck-On Compressor Clutch | $120-$300 clutch kit + 2 hrs | Hard | Medium | 10% |
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Low-side pressure is dropping below freezing. Usually low refrigerant, but can also be a clogged expansion valve or moisture in the system.
Not immediately, but it means the system is not running at the right pressure. The compressor wears faster and you may damage the evaporator over time.
Turn off the AC. The system will not cool properly anyway and continued operation can damage the compressor. Get it diagnosed within a few days.
If the cause is low charge, yes. If the cause is moisture or a clog, recharging just delays the failure. Have it diagnosed first.
A shop will pull vacuum and hold it. A system with moisture takes much longer to reach deep vacuum and the moisture must be boiled out before recharge.
Recharge: $150-$250. Drier replacement: $200-$400. Expansion valve replacement: $300-$600. Diagnosis points to which one you need.
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