Our airconditioner gives a "C5" Error when in heating mode, which is annoying as Canberra gets quite cold in winter.
Looking at the list of error codes, it shows that C5 is a thermistor failure on the indoor gas pipe. Opening the indoor unit and measuring the resistance of the thermistor shows it in the ~2.8MOhm range, well outside what it should be.
I did find a replacement on Ebay, but it was painfully expensive, at around AUD$45. Fortunately, there is a record of the resistance values for the R3T gas pipe thermistor. The service manual confirms this is a 20K thermistor. Plugging the values for 25C and 50C from the table into an online calculator indicates that the thermistor has a Beta of 3950. A quick search on Aliexpress gives us a suitable waterproof thermistor with long leads, a set of 10 for less than AUD$10.
The connector appears to be JST EH 2 pin plug. I've found some on Aliexpress, and will update this once I confirm they will mate with the connector on the PCB.
Update: The parts arrived and replacing the thermistor worked. The new thermistor was a bit smaller than the original, so it was too loose in the clip. I ended up tucking it into the insulation on the gas pipe instead.