HVAC Error Code C4 — What It Means and How to Fix It

What Does This Error Mean?

HVAC error code C4 typically means the condensate drain float switch has tripped. Water in the indoor unit's drain pan has risen too high. The unit shuts down automatically to prevent water from overflowing and damaging your ceiling or floors.

Affected Models

Ceiling cassette AC units
Mini-split indoor units with drain pans
Mitsubishi ceiling cassettes
Daikin cassette units
Generic split systems

Common Causes

The condensate drain line is blocked by algae, dirt, or debris
The condensate drain pump has failed and is not moving water away from the unit
The drain pan is cracked or misaligned, causing water to pool incorrectly
The drain float switch itself is stuck or faulty and is falsely triggering
Excessive humidity is producing more condensation than the drain can handle

How to Fix It

Turn off the AC unit immediately when C4 appears. Check the drain pan below the indoor unit for standing water.

Standing water confirms the drain is blocked. If the pan is dry, the float switch may be stuck or faulty — which is still worth investigating.
Locate the condensate drain line outlet — usually a small PVC pipe that exits through a wall or drains into a household drain. Clear any visible blockage at the outlet end.

Algae buildup at the drain outlet is a very common cause of C4. Pulling out any visible growth often immediately allows water to drain.
Pour a mix of 1 part bleach to 10 parts water into the drain access port on the indoor unit. Let it sit for 15 minutes then flush with clean water.

Bleach kills algae and breaks up soft clogs inside the drain pipe. This should be done as a preventive measure every 3–6 months.
If accessible, use a wet-dry shop vacuum on the drain outlet to suction out the blockage from the outside end of the drain pipe.

This is the most effective DIY drain clearing technique. It pulls the clog toward you rather than pushing it deeper.
Once the drain is clear and the pan water has drained, reset the unit by turning the circuit breaker off for 5 minutes, then back on.

The float switch resets automatically when the water level drops. If C4 returns immediately after resetting, water is still present or the float switch is stuck — call a technician.

When to Call a Professional

If clearing the drain does not resolve C4, call an HVAC technician. A failed drain pump needs replacement. A cracked drain pan needs professional assessment. Never disable or bypass the float switch — it prevents water damage that can cost thousands of dollars to repair.

Frequently Asked Questions

How do I prevent C4 from happening again?

The best prevention is regular drain maintenance. Once a month, pour a cup of diluted bleach (or a dedicated AC drain tablet) into the drain access port. This kills algae before it can accumulate into a blockage. Also replace your air filter regularly — a dirty filter causes more condensation.

Why does my AC drain block up so often?

Units in humid climates produce a lot of condensation. That water carries microscopic dust and organic material into the drain line. Algae and mold feed on this material and grow inside the dark, moist drain pipe. Regular treatment with bleach or commercial drain tabs prevents the growth from ever becoming a blockage.

Can I still use the AC with C4 showing?

No — do not try to override or ignore C4. The float switch is protecting your home from water damage. An overflowing drain pan in a ceiling-mounted unit can ruin drywall, insulation, electrical wiring, and flooring below. Fix the drain blockage first, then resume using the AC.