What causes Mitsubishi Mini Split P4?

The drain float switch connector (CN4F) on the indoor control board is loose, not fully inserted or unplugged, often after service or installation work. A defective indoor controller board that doesn't read the float switch circuit. On units with a drain sensor (DS) instead of a float switch, a disconnected CN31 connector or a faulty drain sensor. Drain pump or drain line problems found during the checks Mitsubishi lists for P4 (on units with a pump, a P5 code usually points to a real overflow).

How do I fix Mitsubishi Mini Split P4?

Write down the code and, if you have a wired remote, the exact check code shown; on a wireless remote, P4 is 4 blinks or beeps during self-diagnosis. Think about whether anyone recently worked inside the indoor unit (cleaning, a pump install, a board swap). A connector knocked loose during that work is the most common cause Mitsubishi lists, so call that installer back first. Check that the condensate drain line isn't clogged or kinked and that water drains freely; a working drain is part of Mitsubishi's P4 checks. Turn the system off at the breaker for a few minutes and restart it. If P4 comes back, stop resetting it and call a licensed HVAC technician. Don't open the indoor unit's electrical box or bypass the float switch yourself; it's a safety device that protects your ceiling and walls from water damage.

When should I call a pro for Mitsubishi Mini Split P4?

Check that the float switch connector CN4F (or the drain sensor connector CN31 on drain-sensor units) is fully seated on the indoor controller board, then power the unit back on and check again. Per Mitsubishi's OCH637 table, operate the unit with connector CN4F short-circuited as a test; if the abnormality reappears with CN4F shorted, replace the indoor controller board. Remove the short when done, since the float switch must stay in service. On drain-sensor (DS) units, measure the resistance of the indoor thermistors, as Mitsubishi's OCH636E table directs. Measure the resistance among the drain pump terminals with a multimeter, check that the drain pump runs, and check the drain function. Under warranty, go through the installing contractor or a Mitsubishi Electric Diamond Contractor.

Code: P4🔧 Mitsubishi Mini Split

Mitsubishi Mini Split Error Code P4: Float Switch or Drain Sensor Error (Meaning, Causes, Fixes)

Q: What does Mitsubishi Mini Split P4 mean?

P4 on a Mitsubishi Electric mini split (Mr. Slim indoor units) is a condensate drain safety fault on the indoor unit. On most current indoor units Mitsubishi calls it "float switch connector open": the control board can't see the drain float switch, usually because its connector (CN4F) is loose or unplugged, or the indoor control board is bad. On some indoor units that use a drain sensor instead, the same P4 code means "abnormality of drain sensor (DS)" on connector CN31. Wireless remotes show it as 4 blinks or beeps. Checking the connector and the drain is a reasonable first step, but board and pump work belongs to an HVAC technician. Mitsubishi Electric's indoor unit service manual OCH637 (PKA-A12HA7/PKA-A18HA7 wall units) lists P4 as "Float switch connector (CN4F) open" and as 4 blinks or beeps of the OPERATION INDICATOR on the wireless remote self-diagnosis. Its troubleshooting table describes the detection: the error is triggered when the connector of the drain float switch is disconnected, meaning pins 3 and 4 of connector CN4F are not short-circuited, and it is checked constantly during operation. The listed causes are a contact failure (insert failure) of the connector and a defective indoor controller board. Mitsubishi's outdoor unit service manual OCH636E (PUZ-A/PUY-A NKA7/NHA7) lists the same "float switch connector open (FS)" P4 and adds checks of the drain pump and drain function. The same OCH636E manual's second error table, and Mitsubishi Electric Singapore's Mr. Slim error chart, also list P4 as "Abnormality of drain sensor (DS)" with connector CN31 on the indoor controller board, because some indoor units use a thermistor-type drain sensor instead of a float switch. Either way, P4 is about the drain safety device and its wiring; a drain that actually overflows is a separate code, P5 (drain overflow protection or drain pump error).

Reviewed 2026-10-24 · Based on 3 manufacturer & service sources

What the code means

Mitsubishi Electric's indoor unit service manual OCH637 (PKA-A12HA7/PKA-A18HA7 wall units) lists P4 as "Float switch connector (CN4F) open" and as 4 blinks or beeps of the OPERATION INDICATOR on the wireless remote self-diagnosis. Its troubleshooting table describes the detection: the error is triggered when the connector of the drain float switch is disconnected, meaning pins 3 and 4 of connector CN4F are not short-circuited, and it is checked constantly during operation. The listed causes are a contact failure (insert failure) of the connector and a defective indoor controller board. Mitsubishi's outdoor unit service manual OCH636E (PUZ-A/PUY-A NKA7/NHA7) lists the same "float switch connector open (FS)" P4 and adds checks of the drain pump and drain function. The same OCH636E manual's second error table, and Mitsubishi Electric Singapore's Mr. Slim error chart, also list P4 as "Abnormality of drain sensor (DS)" with connector CN31 on the indoor controller board, because some indoor units use a thermistor-type drain sensor instead of a float switch. Either way, P4 is about the drain safety device and its wiring; a drain that actually overflows is a separate code, P5 (drain overflow protection or drain pump error).

Likely causes

The drain float switch connector (CN4F) on the indoor control board is loose, not fully inserted or unplugged, often after service or installation work
A defective indoor controller board that doesn't read the float switch circuit
On units with a drain sensor (DS) instead of a float switch, a disconnected CN31 connector or a faulty drain sensor
Drain pump or drain line problems found during the checks Mitsubishi lists for P4 (on units with a pump, a P5 code usually points to a real overflow)

Safe DIY checks

⚠ Before you start

Unplug the appliance or switch off its breaker before opening panels.

Write down the code and, if you have a wired remote, the exact check code shown; on a wireless remote, P4 is 4 blinks or beeps during self-diagnosis.
Think about whether anyone recently worked inside the indoor unit (cleaning, a pump install, a board swap). A connector knocked loose during that work is the most common cause Mitsubishi lists, so call that installer back first.
Check that the condensate drain line isn't clogged or kinked and that water drains freely; a working drain is part of Mitsubishi's P4 checks.
Turn the system off at the breaker for a few minutes and restart it. If P4 comes back, stop resetting it and call a licensed HVAC technician.
Don't open the indoor unit's electrical box or bypass the float switch yourself; it's a safety device that protects your ceiling and walls from water damage.

🛠 When to call a pro

Check that the float switch connector CN4F (or the drain sensor connector CN31 on drain-sensor units) is fully seated on the indoor controller board, then power the unit back on and check again.
Per Mitsubishi's OCH637 table, operate the unit with connector CN4F short-circuited as a test; if the abnormality reappears with CN4F shorted, replace the indoor controller board. Remove the short when done, since the float switch must stay in service.
On drain-sensor (DS) units, measure the resistance of the indoor thermistors, as Mitsubishi's OCH636E table directs.
Measure the resistance among the drain pump terminals with a multimeter, check that the drain pump runs, and check the drain function.
Under warranty, go through the installing contractor or a Mitsubishi Electric Diamond Contractor.

Parts that commonly fix it

Always match the part number to your exact model.

⚙

Mini split indoor control board (exact part for your indoor model)

Mitsubishi lists a defective indoor controller board as a P4 cause; match the part number to your indoor unit and have a technician confirm it first.

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⚙

Mini split condensate drain pump

Mitsubishi's P4 checks include the drain pump's terminal resistance and whether it runs.

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⚙

Digital multimeter

Used to measure drain pump terminal resistance and thermistor values in Mitsubishi's P4 checks.

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Sources

Last reviewed 2026-10-24. Always confirm against the manual for your exact model number.

Mitsubishi Mini Split Error Code P4: Float Switch or Drain Sensor Error (Meaning, Causes, Fixes)

What the code means

Likely causes

Safe DIY checks

🛠 When to call a pro

Parts that commonly fix it

Mini split indoor control board (exact part for your indoor model)

Mini split condensate drain pump

Digital multimeter

Related codes

Sources