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Air Conditioner Indoor and outdoor coils: What It Does and Signs It's Failing

The finned copper or aluminum heat exchangers. The indoor coil sits behind the filter in the indoor unit; the outdoor coil wraps around the outdoor unit. They carry refrigerant and pass heat to or from the air.

How it works

Air from the fans moves across the fins. In cool mode the indoor coil absorbs heat and the outdoor coil releases it; a heat pump reverses this.

Signs it is failing

• Dust, leaves or grass packed into the outdoor fins
• A dirty, matted indoor coil behind the filter
• High pressure or overheating codes
• Weak cooling
• Frost or ice on the coil

Dirty coils are a leading cause of high-pressure and overheating codes. DOE recommends checking the indoor coil every year and cleaning it as needed, and keeping plants trimmed back at least 2 ft (0.6 m) from the outdoor unit.

Can you replace it yourself?

Cleaning is yours; replacement isn't. With the breaker off, hose loose dirt off the outdoor coil with a gentle spray (never a pressure washer, which flattens the fins) and keep plants trimmed back. Brush or vacuum dust off the indoor coil face behind the filter, gently, because the fins are sharp and bend at a touch. A leak, badly damaged fins or a coil replacement means refrigerant recovery and recharge, which in the US requires EPA Section 608 certification.

Buying the right part

If you use a coil cleaner, pick one labeled for your type of unit and follow its directions; a deep clean of a wall unit's coil and blower is a service job. Replacement coils are bought and fitted by a tech.

Error codes linked to the indoor and outdoor coils

• E1High pressure protectionGree
• CH61Unit overheatingLG
• U1 / UdHigh pressure or overheat protectionMitsubishi Electric

Last reviewed 2026-10-19.