Error Code U2

[aodhan]

Installed a new Mitsubishi Electric A/C unit, Outdoor model no. PUHZ-P140VHA3.... unit runs for a few minutes on heating mode and then goes on standby and then back to heating... discharge pressure seems to rise very quick to approx 26/28 bar... controller then shows up fault code U2... calling back to site on monday morning and looking for help on what steps i should carry out first to find the problem...
all help greatly appreciated...

[stufus]

Off the top of my head U2 is abnormal discharge temp.
If its a new install check the obvious first,
Did it require additional refrigerant for pipe run ?
Was refrigerant added in correct quantity ?
Are both Valves open?
Was it pressure tested and vacced correctly ?
Does it run in cooling test mode?

Just a few suggestions

Best of luck

Stu

[stufus]

This might help you out a bit

U2
High discharging temperature
Abnormal if discharge temperature
thermistor (TH4) exceeds 125: or 110:
continuously for 5 minutes.
Abnormal if during defrosting discharge
temperature thermistor (TH4) exceeds
110: continuously for 30 minutes.
1 Overheated compressor
operation caused by insufficient
refrigerant
2 Defective thermistor
3 Defective outdoor controller
board
4 Defective operation of linear
expansion valve
1 Check intake super heat.
Check leakage of refrigerant.
Charge additional refrigerant.
23 Turn the power off and check if U3 is dis-
played when the power is turned ON again.
When U3 is displayed, refer to “Judgement
and action” for U3.
4 Check linear expansion valve.
Refer to 8-5.
U3
Open/short circuit of discharge
temperature thermistor (TH4)
Abnormal if open (3: or less) or short
(217: or more) is detected during
compressor operation.
(Detection is inoperative for 10 seconds to
10 minutes of compressor starting process
and for 10 minutes after or during defrosting.)
1 Disconnection or contact
failure of connector (TH4) on
the outdoor controller circuit
board.
2 Defective thermistor
3 Defective outdoor controller
circuit board
1 Check connection of connector (TH4) on the
outdoor controller circuit board.
Check the lead wire for thermistor (TH4).
Refer to 8-6.
2 Check resistance value of thermistor (TH4) or
temperature on LED display.
(Thermistor/TH4: Refer to 8-5.)
(SW2: Refer to 8-7.)
3 Replace outdoor controller board.

[aodhan]

Stu

Its a brand new unit... pipe length is only approx. 12mts... unit was pressure tested and vaccumed out overnight so all ok that way... didnt check unit on cooling mode... had unit on heating mode only as shop was very cold 8 degrees return air temp on controller and outside temp was approx -1 degree.... i take it unit was going on stand-by until it builds up heat....

[stufus]

Hey Aodhan

Initially the stand-by scenario is to allow coil temp to rise before starting fans.
But you need a discharge temp in excess of 125c for 5 mins to generate a U2 fault.
This is just a shot in the dark but did you purge the pipe work through the indoor and back to the outdoor before pressure test to establish a clear circuit IE no blockages ,
The only reason i ask is on a few occasions ive been called to sites For hp/lp faults to find someone has either kinked a pipe or left one of the flare blanks (bonnets) in place.

On the otherhand it could be a faulty thermister TH4

[regcooldude]

if it's new check the sensor has not come out of its pipe holder. i've had a few like that, sensor on pipework just above compressor

[stufus]

If the discharge sensor was out of its pocket it would read a lot lower than the 100 + degrees to trip the unit ,
Although i do agree with the fact that they regularly turn up out of position ,even disconnected from the board from time to time