Alarm At Idle, Silent As RPM Increase

[Hullphar]

Nissan outboard 1996 90 hs alarm goes off in control box at low rpm. Throttle up and it goes off. In repair shop twice to no results

 

[pvanv]

You did not mention model. Year is irrelevant. Assuming you have an NS90A. You can DL your owners manual from https://tohatsu.com/tech_info/manuals.html
Page 62 shows the alarms for the 90A. Do you get any indicator lights?
You can use an IR thermometer to see if you are running hot. If your water pump is worn, it might not pump well at idle. How strong is the telltale stream?

 

[Sea Rider]

Will be good to know if that 23 year oldie motor was used on fresh or salt water along thermo and heat probe condition if doing their work as they should or already being faulty . If you can place your hand firm on the powerhead and hold it there for say 3-5 seconds soon after the warm up time is over will be OK, if need to remove it inmediately the motor is experiencing overheat issues that will need to be addressed properly.

Happy Boating