Thermostat – Fits Tohatsu/Mercury/Evinrude 2-6 HP 4-Stroke (Replaces 3H6-01030-0, 804540)

$54.58 $87.00

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Thermostat – Fits Tohatsu/Mercury/Evinrude 2-6 HP 4-Stroke (Replaces 3H6-01030-0, 804540) is backordered and will ship as soon as it is back in stock.

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Product Details

This Thermostat – Fits Tohatsu/Mercury/Evinrude 2-6 HP 4-Stroke (Replaces 3H6-01030-0, 804540) is engineered for small 4-stroke outboard engines from Tohatsu, Mercury/Mariner, and Evinrude. It ensures your engine maintains the correct operating temperature for optimal performance and longevity, covering various models from 2002 and later.

It serves as a direct replacement for Tohatsu 3H6-01030-0/345-01030-0, Mercury 804540/853704/8537041, Evinrude 5040958, and Sierra 18-3638.

Designed for engine block mounting.
Regulates temperature at the standard OEM specification of 52°C (126°F).
Includes one thermostat ready for installation.

Part Number Matrix

Use this matrix to confirm equivalent references across OEM and aftermarket brands.

Brand	Part Number
Tohatsu	3H6-01030-0
Tohatsu	345-01030-0
Mercury	804540
Mercury	853704
Mercury	8537041
Evinrude	5040958
Sierra	18-3638

Engine & Model Compatibility

Confirm your engine model/year in the tables below.

For Mercury/Mariner models, this thermostat fits 4-stroke carbureted engines from 2 HP to 6 HP (2002 and up) and certain international 25-30 HP models. Check your serial number to confirm fitment.

Brand	Model	Rated HP	Stroke	Years	OEM Part No.	Notes
Mercury	Carbureted	2	4-Stroke	0R131842 & Up	804540
Mercury	Carbureted	2.5	4-Stroke	0R131842 & Up	804540
Mercury	Carbureted	3.5	4-Stroke	0R131842 & Up	804540
Mercury	Carbureted	4	4-Stroke	0R000001 & Up	804540
Mercury	Carbureted	5	4-Stroke	0R000001 & Up	804540
Mercury	Carbureted	6	4-Stroke	0R000001 & Up	804540
Mercury	International Models	25-30	4-Stroke		804540	2-Cylinder, 430cc

For Tohatsu/Nissan engines, coverage includes MFS models from 2 HP to 6 HP between 2002 and 2014.

Brand	Model	Rated HP	Stroke	Years	OEM Part No.
Tohatsu	MFS2/MFS2A/MFS2B	2	4-Stroke	2006–2014	3H6-01030-0
Tohatsu	MFS2.5/MFS2.5A/MFS2.5B	2.5	4-Stroke	2009–2014	3H6-01030-0
Tohatsu	MFS3.5/MFS3.5A/MFS3.5B	3.5	4-Stroke	2006–2014	3H6-01030-0
Tohatsu	MFS4	4	4-Stroke	2002–2014	3H6-01030-0
Tohatsu	MFS5	5	4-Stroke	2002–2014	3H6-01030-0
Tohatsu	MF6	6	4-Stroke	2002–2014	3H6-01030-0

For Evinrude outboards, this part fits B, E, and J series 4-stroke models between 3 HP and 6 HP from 2002–2012.

Brand	Model	Rated HP	Stroke	Years	OEM Part No.
Evinrude	E3, B3	3	4-Stroke	2012	5040958
Evinrude	J3	3	4-Stroke	2002–2006	5040958
Evinrude	E4, B4	4	4-Stroke	2012	5040958
Evinrude	E6, B6	6	4-Stroke	2012	5040958

Technical Specs

Product Type: Thermostat
Installation: Engine Mount
Operating Temperature: 52°C (126°F)
Temperature Rating: Standard OEM spec
Housing Material: Metal (brass/steel)
Housing Color: Silver
Other Technical Attributes: Direct OEM replacement design

What’s in the Box

1 x Thermostat (3H6-01030-0 / 804540)

Installation Notes

Ensure the engine is completely cool before starting.
Disconnect the battery to prevent accidental starting.
Locate the thermostat housing on the engine block, referring to your service manual if needed.
Place a drain pan under the housing and remove the cover bolts.
Carefully remove the old thermostat and gasket. Note its orientation.
Clean both the housing and cover surfaces thoroughly, removing all old gasket material.
Install the new thermostat in the same orientation as the old one.
Fit the new gasket and replace the housing cover, tightening the bolts to the manufacturer's specified torque.
Reconnect the battery.
Start the engine and check for leaks around the housing as it warms up.

Troubleshooting & FAQ

How do I know if my thermostat has failed?

Common signs of a failed thermostat include engine overheating at low speeds, or taking too long to reach operating temperature. A thermostat stuck open will cause the engine to run too cool, leading to poor performance and carbon buildup. A thermostat stuck closed will cause rapid overheating.

What temperature does this thermostat open at?

This thermostat is designed to open at 52°C (126°F), which is the standard operating temperature for the compatible engine models, ensuring efficient combustion and engine longevity.

How do I confirm this fits my specific engine model?

The best way is to compare your old part's number to the ones listed in the Part Number Matrix. Additionally, check your engine's model, HP, and year against the compatibility tables provided. If in doubt, consult your engine's service manual or a qualified marine mechanic.

What happens if I run the engine without a thermostat?

Running without a thermostat is not recommended. The engine will run too cold, resulting in incomplete fuel combustion, increased engine wear, carbon deposits, and poor overall performance. The thermostat is critical for maintaining the correct operating temperature.

How often should I replace my outboard's thermostat?

Thermostats don't have a strict replacement interval, but it's good practice to inspect them every 1-2 years, especially if operating in saltwater. Many owners replace them proactively every 2-3 years as part of routine cooling system maintenance.

OEM vs. Aftermarket?

This listing is an aftermarket replacement (not OEM). It is built to OEM-level specifications with comparable materials. The part is manufactured in an ISO 9002 facility and is batch-inspected for consistency and quality.

Shipping speed & cost

We ship worldwide quickly—typical transit is 5–15 days. Standard shipping is free; expedited options will appear at checkout where available.

Warranty & returns

This product is covered by a 1-year warranty against manufacturing defects under normal use. If issues arise, contact our support team with your order details. Returns are accepted within our standard window for unused items in their original condition. Please see our policy page or reach out for instructions.

Safety & Important Info

Verify images and specifications against your original part. If anything differs—even if your model appears compatible—do not purchase.
This is an aftermarket replacement component, not an OEM part.
Professional installation is recommended for safety and performance.

About Sea Sierra

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