Cause of error code A18

Max Temp Error.

Solution for error code A18

1. Disconnect harness from High Temperature Supply Switch and Control Module J13. Continuity from J13-5 to Switch 1? Continuity from J13-6 to Switch 3?
Yes: -> 2.
No: Replace wire(s)

2. Is supply water temperature less than 185°F (91°C)?
Yes: -> 3.
No: -> 5.

3. Is High Temperature Supply Switch open (no continuity between terminals 1 and 3)?
Yes: Replace High Temperature Supply Switch. 
No: -> 5.

4. Measure resistance across High Temperature Supply Switch terminals 2 and 4? Does resistance match supply water temperature? 
Yes: Replace Control Module.
No: Replace High Temperature Supply Switch. 

5. Is High Temperature Supply Switch open (no continuity between terminals 1 and 3)?
Yes: Replace High Temperature Supply Switch.
No: Replace Control Module.