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C4 is not an error it's an indication of where in the sequence an error may have occurred. Try turning off the power to the unit by pressing the power ON/OFF button on the remote, wait 5 minutes then turn the power back on and turn on a hot water faucet and see if the error returns. If it does tell me what it is. I can probably post a link to the manual but I need the actual model number of your unit
No you turn the remote off then back on to reset the unit if it is showing an error
If you don't know the model number I can post a manual for an EcoSense heater. While it will be essentially the same there are a few model specific variations. Setting the water temp with the remote is in the manual and should be the same for all EcoSense models