Mitsubishi MLZ-KP09NA 9,000 BTU One-Way Ceiling Cassette Indoor Unit P4 Error Fix: Drain Sensor / Float Switch Error

Error code P4 on the Mitsubishi MLZ-KP09NA 9,000 BTU One-Way Ceiling Cassette Indoor Unit means: Drain Sensor / Float Switch Error. The condensate drain float switch has activated, indicating the drain pan is full or the drain line is blocked. This is a very common issue — the condensate drain line (typically 3/4 inch PVC or vinyl tubing) becomes clogged with algae, mold, or debris over time. The indoor LED blinks 5 times with this fault. Clear the drain line with a wet/dry vacuum or nitrogen flush. Check the drain pan for cracks. Verify the drain line has proper slope for gravity drainage. Install drain line algae prevention tablets.

The most successful fix is "Flush and Repair the Drain Line and Replace Float Switch if Needed" (medium difficulty, ~60 minutes). Estimated DIY cost: $15-$25. HomeMD has 2 community-ranked fixes for this error.

This error is rated critical severity. It is generally not urgent and can often be resolved with a simple DIY fix.

Yes — the top-ranked community fix ("Flush and Repair the Drain Line and Replace Float Switch if Needed") is rated medium difficulty and takes about 60 minutes. Tools needed: T20 Torx screwdriver, Nitrogen flush kit or garden hose, Multimeter.

DIY cost: $15-$25. Professional repair typically costs $150-$220. Many homeowners save money by trying the top-ranked community fix first.

The Mitsubishi MLZ-KP09NA 9,000 BTU One-Way Ceiling Cassette Indoor Unit has 8 other known error codes, including: E1 (Remote Controller PCB Error), E2 (Remote Controller Signal Receiving Error), E3 (Remote Controller Communication Error), E4 (Indoor Unit PCB Processor Error), E5 (Indoor/Outdoor Communication Error (Legacy)), and 3 more.

Error code P4 also appears on 8 other Mitsubishi models: MSZ-EF15NA 15,000 BTU Designer Series (Kirigamine Zen) Wall-Mount Indoor Unit, MSZ-FH12NA 12,000 BTU Hyper-Heat H2i Wall-Mount Indoor Unit, MSZ-FS06NA 6,000 BTU Compact Wall-Mount Indoor Unit, MSZ-GL09NA 9,000 BTU Wall-Mount Ductless Mini-Split Indoor Unit, MSZ-GL12NA 12,000 BTU Wall-Mount Ductless Mini-Split Indoor Unit, SEZ-KD12NA 12,000 BTU Ceiling-Concealed Ducted Indoor Unit, and 2 more. The fix may vary by model, so check each model's specific page for tailored repair steps.