Need help with Mitsubishi Mr Slim ceiling cassette

[miketyler]

Hoping someone here can offer some advice. I installed an 18k btu Mitsubishi Mr Slim mini split unit in the loft above my shop several years ago. The ceiling cassette was a pain to install but the unit has been working well for me.

Last month the inside fan wouldn't come on and flashed a P5 condensate pump error. I checked the line and it was clear. I could hear the pump motor running but couldn't get to it unless I dropped the pan. I dropped the styrofoam pan, cleaned it and reinstalled. The P5 error code is now gone but the fan cycles off and on every 30-40 seconds. It is cooling but seems like the controller is stuck in a loop. Any thoughts on this? I don't see anything in the manual that describes this. I've checked and the PCB is obsolete and can't be sourced anywhere.

 

[WildBill]

Double-check for any loose connectors? I accidently unplugged the thermistor connection on mine once when cleaning, it got pretty upset. Is there a sensor/float switch for the drain that's doing something funny? Did you kill the breaker for a couple mins to completely reset it? Any error codes blinking on the mainboard in the outside unit?

 

[miketyler]

Hey Bill -
Will have to check for loose connectors. I did have to pull the inside board to drop the pan and all seems kosher on reassembly. There is a round float switch below the pump motor that I was very careful to clean and ensure it had free movement. I killed the breaker before I started this. I assume the fan is getting instruction from the inside board but will check the outside LED's. It's been a great unit and I'd hate to pull it out to replace it. It is odd, it cycles the fan to full rpm, cuts off then restarts again.

 

[WildBill]

In my limited experience they pretty much always flash a code on the outside board if there is a real issue, anything weird without an outside code comes down to a sensor/connector/wiring issue.

 

[miketyler]

I did initially get this P5 code at the console but is clear now. I'll pop the cover on the condensing unit and check the LED's.

 

[chinboys]

Google AI...
The P5 error code on a Mitsubishi Electric air conditioning unit indicates a problem with the drain pump, which is responsible for removing excess water from the unit. Some possible causes of this error code include:

• A defective drain pump
• Electrical issues with the wiring or connections to the drain pump
• Blockages in the drain line

Perhaps the condensate level float sensor is mechanically impinged or bad?

 

[miketyler]

Thanks but P5 error code is now cleared after I disassembled and cleaned unit. The unit is cooling ok but having an inside fan cycling problem as I posted earlier. No error codes at the console.

 

[miketyler]

@WildBill I got to thinking about reset. I powered the system off at the breaker while I was doing the cleaning. That was probably 2-3 hours. Last night I powered off the system and then cut power at the breaker and left it down until a bit ago. Restored power at the breaker and then powered on the unit and so far is cooling normally with no cycling. Hopefully this is fixed now.

 

[miketyler]

Well that was short lived. Once it got to it's set point, the inside fan began cycling repeatedly like before. Normally when its maintaining temp, the vents remain open but the fan RPM slows way down. You can't tell that it's running unless you open the filter door.

 

[American Locomotive]

I would suspect a thermistor or a wire that wasn't connected properly.