Sensor failure started heater and displays sensor failure and will not utrn on

sounds like the temp sensor is bad

Year, model, and size of the heater.
What is the error message exactly?

SN3 - Temperature Sensor Failure.
Either the sensor is bad or it wasn't plugged back in correctly.

SF is the code for Temperature sensor input failure. check wiring that goes to temperature sensor for brakes or cuts, sensor is located on right lower side of heater where water lines go in. you have to remove the 6 screws to take the side panels off above and below the water lines. sensor is visible then.follow wires back to control board and make sure they are securely pluggin in to board and all broken wires fixed or replaced. If no broken wires or loose connections, Replace temperature sensor.

CODE DISPLAYED FAULT REMEDY 11 Ignition Failure Check that gas is turned ON. Service call if repeated. 12 Flame Failure Check that gas is turned ON. Service call if repeated. 14 Overheat Clean filter. Service call if repeated. [1] 16 Room Overheat Lower room temperature to less than 104°F (40°C). 31 32 Room Temperature Sensor Faulty Service call. 33 34 Overheat Temperature Sensor Faulty Service call. 49 Sensor Breakdown Service call. 53 Sparker Failure Service call. 61 Combustion Fan Failure Service call. 70 Faulty ON/OFF Switch Service call. 71 Faulty Solenoids Service call. 72 Faulty Flame Rod Service call. 73 Communication Error Turn heater OFF, then ON again. 99 Flue Block Check around the flue terminal for blockage. [2] [1] Remove any obstructions. The flue needs to be kept clear to expel exhaust gases. If the appliance fails to operate contact a qualified service agency. [2] If the fusible link needs replacement, it must be done by a qualified service agency. In addition, the cause of the overheat needs to be determined. The fusible link is a one use safety device that breaks to shut off the appliance.

sns occurs when sensor reads temps under 36 degrees or above 110 degrees. check temp of of water to verify.....may need a new water sensor