Mercury Mariner 200, 225 Optimax Outboards Service Manual, 90-855348, Page 202Get this manual
Problem: Warning Horn Stays on When Ignition Key is Turned to "ON" PositionPossible Cause Engine overheat sensor
Corrective Action
If horn sounds continuous signal, the engine overheat sensor may be faultyDisconnect overheat sensor and turn ignition key to "ON" positionIf horn still sounds the engine overheat sensor is OK
Tan wire between warning horn and engine overheat Check wire for shortssensor is shorted to groundElectronic Control Module (ECM) Check connectionsreplace ECM