Keep your Mercury running like new with this genuine THERMAL SWITCH, (Normally Open 130-110 degrees). Built to the same exacting standards as your original equipment for worry-free boating.

Engineered to Mercury’s rigorous OEM standards, this switch ensures reliable operation across all engine RPM ranges. Part number 866089 is manufactured to exact Mercury Marine factory specifications for precision installation and optimal performance. Constructed from premium marine-grade materials designed to withstand harsh saltwater and freshwater environments.

Why risk your Mercury engine with questionable aftermarket parts? Genuine part 866089 is built to last, backed by Mercury Marine’s decades of marine engineering expertise and proven reliability.

Compatible Mercury Marine Engines

FITS: 4-5HP MERCRUISER

• Fits MERCRUISER 4-5HP engines
• Compatible HP: 4HP, 5HP
• Covers 19 serial number ranges
• Configurations: Outboard
• OEM genuine Mercury Marine part

COMPLETE SERIAL NUMBER COMPATIBILITY:

4HP Models

• 4HP OUTBOARD (Mercruiser): 0M615000-0W300012, 0W300013-0W309999, 0W310000-0W649999, 0W650000-1A063239

5HP Models

• 5HP OUTBOARD (Mercruiser): 0M600000-0W300009, 0W300010-0W309999, 0W310000-0W649999, 0M600000-0W300754, 0W300755-0W309999, 0W310000-0W649999, 0M317000-0W090899, 0W090900-0W689999, 0M317000-0W090899, 0W090900-0W689999

Need help finding your serial number? Check our serial number location guide for step-by-step instructions on locating it on your Mercury engine.

Verification & Support

Want to verify compatibility for part 866089? Use our parts finder tool for instant confirmation. Still have questions? No problem – use the contact buttons at the bottom of this page to reach us by text, call, or email

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