LG split system cooling problem

[jmw76]

Hi All,
I have been trying to track down an issue with an older LG split A/C system (6kw domestic).
Model number is LSZ182VM-4.
The system does not work in cooling mode and throws a C6 error on the indoor unit and flashes the diagnostic LED 6 times on the outdoor unit.
The system works perfectly in heating mode.
9 months ago the system had the outdoor unit control board replaced (supposedly for the same fault). The unit has since been operated during the colder months as a heater successfully. Now in the warmer months it has been discovered to be faulty again.

It appears in cooling mode that the compressor does not start. It appears to try a number of times and then just shuts down with the error.

I have checked the voltage on the 3 phases for the compressor (when operating in heating mode) and they are all equal and change in proportion to compressor speed as expected.
For this reason, I do not believe that the invertor or compressor are the problem (otherwise it would not work in heating mode??).
I have checked the resistance of all (at least I think I found them all) thermistors, both indoor and out, and I think they are ok (although I do not have good specs for their nominal values). The indoor values are 5K and 10k for the room temp and pipe temp sensors . The outdoor vales are 10k and 1k (???) for the ambient and pipe sensors and 150-200k for the compressor sensor.

I am not that familiar with this unit and I find the diagnostic codes a bit misleading.

Any advice would be greatly appreciated.

Peter.

[frank]

Not too familiar with LG units - repaired a few but that's about it.

Looking on the net, the C6 error refers to a sensor fault - see below.

Some makes use the indoor/outdoor coil sensors at different times (heating/cooling) so my first suggestion would be to check the outdoor coil sensor which may be out of calibration giving a wrong resistance reading to the main pcb resulting in the error.

Wall Units (Single split)C1 or CH1: A fault with the indoor temperature sensor, open or closed circuit
C2 or CH2 2: A fault with the indoor temperature sensor, open or closed circuit
C4 or CH4 4: There’s an issue with the heat sink temperature sensor, open or closed circuit
C5 or CH5 5: The issue is with the communication between indoor and outdoor units
C6 or CH6 6: The inverter DC power circuit is experiencing excessive voltage(can be indoor pipe outlet sensor, or outdoor condenser sensor on some models)
C7 or CH7 7: There’s excessive compressor current present
C8 or CH8 8: The indoor fan has stopped rotating
C9 or CH9 9: The outdoor fan has stopped rotating
CA 10: The discharge temperature has exceeded 130 degrees or there is a fault with the discharge thermistor
CC 2 + 1: EEPROM error
CD 3 + 1: The inverter module has encountered an error
Po: System in Power Mode – this is not a fault
Lo: System in test mode – this is not a fault

[jmw76]

Hi Frank,

Thanks for the response. I will carefully check out all of the thermistors. The one on the outdoor unit that is reading ~1kohm at 25C is puzzling. I expected it should be 5k or 10k but can not find any decent specs on what it should be. I have also realised that I only identified 2 sensors on the indoor unit. I suspect that there is a 3rd that I did not find at the time. One of the pipe sensors?

Anyone else have any othe clues or has seen this problem before?

Peter.

[frank]

Doing a little more research I came across this:

LG Split System Air Conditioner Error code = C6 or CH06
LG AC Fault Definition = Excessive current at inverter DC power circuit – DC Peak Power
Flashing Light = 6
Display = Indoor Outdoor
Indoor Operation = SHUTDOWN

It looks like there may be a fault with your compressor. Here's a link (https://removeandreplace.com/2017/06/06/lg-split-system-air-conditioner-error-codes-troubleshooting-maintenance/) to the webpage

[jmw76]

Hi Frank,

Thanks for your response.
But with respect, I am still confused. Why would the compressor successfully run in heating mode but fails to start in cooling mode? Clearly the compressor and invertor actually work. I did think that maybe there was an overpressure issue and the compressor locks up with liquid in it. But I do not understand why the static system pressure would be any different between heating and cooling.

I think the thermocouples are the best area to investigate further.
Hopefully, I will get a chance to investigate further over the next few days.

Cheers Peter.

[seanf]

Does it run at all in cooling mode, like if the units been left off overnight and started for the first time in the morning?

[katehens]

Hi Jmr,

I will also advise you check the compressor. Many at times when the compressor is weak, it might still be working well for heat and not work well for cooling. I use LG and I'm familiar with this kind of issue.

[jmw76]

Hi Guys, Thanks for your feedback. I have not really had a chance to investigate further at this stage. Just about to get back into it.
Some responses to the unanswered questions:

1) The unit does not run at all in cooling mode. The compressor does not start up. After some time (several minutes) it just throws up the C6 error on the indoor unit and flashes the LED on the outdoor unit 6 times in repetition.

2) The compressor and inverter seem to run properly in heating mode. the resistances of the compressor windings seem to check out ok and the drive voltages from the inverter board look ok (the board was replaced 9 months. but probably did not need replacing). I am not sure that I understand how the compressor will work in heating and not in cooling. Is it not performing the same function in both applications? It is not like it starts and then fails some time later due to high current.

3) Does anyone know what the resistance values of all the sensors should be at ambient temperature?
I have measured values of 1kohm (actually more like 900ohm), 5kohm, 10kohm and 200kohm. From documentation I have found I believe the 5, 10 & 200k values are most likely correct. The 1k value looks to be suspect.

4) can someone confirm how many sensors (thermistors) are on the indoor unit? When I looked, I only found 2. I suspect that there might actually be 3. One room and one on the inlet and outlet pipes.

Thanks Peter.