Engine Coolant Thermostat Kit, L5P Duramax | 2017-2026

If your L5P Duramax isn't reaching operating temperature or is running hot, a stuck thermostat could be the cause. The Merchant Automotive Engine Coolant Thermostat Kit fits 2017-2026 Chevy Silverado and GMC Sierra 2500HD and 3500HD trucks with the 6.6L L5P Duramax diesel. It includes both genuine GM thermostats and their O-ring seals.

Features

• Genuine GM 185 degree thermostat (12680456)
• Genuine GM 180 degree thermostat (12680457)
• Two thermostat O-ring seals included
• Replaces both thermostats as a pair in one order

What's Included

• (1) GM 185 degree thermostat (12680456)
• (1) GM 180 degree thermostat (12680457)
• (2) Engine Coolant Thermostat O-rings, replaces GM 98280281

When to Replace

• Stuck open: coolant is always flowing to the radiator, so the truck takes a long time to reach operating temperature, if it reaches it at all.
• Stuck closed: the thermostat won't open to let coolant flow to the radiator. This usually leads to overheating, which can cause serious engine damage.
• Emission system: proper engine temperature matters on modern diesels. DPF regeneration may not run if the engine is not reaching full operating temperature.

Before You Order

• L5P 2017-2026 pickups only. For 2001-2010 trucks, see the 2001-2010 Thermostat Kit. For 2011-2016 LML trucks, see the LML Thermostat Kit.
• Coolant is not included. The cooling system must be drained and refilled for this job.
• Housing and bypass pipe seals are sold separately (see Related Parts).

Installation Tips

• Replace both thermostats at the same time.
• Drain the coolant before removing the thermostat housing.
• Install new O-rings on both thermostats.
• Inspect the thermostat housing gasket and bypass pipe seals, and replace them if they are worn or damaged.
• Refill and bleed the cooling system, then check for leaks.

Related Parts

• Engine Coolant Thermostat Housing Gasket, 2017-2026
• Thermostat Bypass Pipe O-ring, 2017-2026
• Thermostat Bypass Pipe Gasket, 2017-2026
• Wehrli Thermostat Housing, L5P Duramax
• Radiator Coolant Surge Tank, 2017-2019
• Surge Tank Overflow Side Pressure Cap
• Merchant Automotive Billet Coolant Block Off Plate
• Coolant Analysis Kit

Frequently Asked Questions

Should I replace both thermostats?
Yes. We recommend always replacing the thermostats as a pair. This kit includes both.

Are the seals included?
The kit includes two thermostat O-rings. The thermostat housing gasket and bypass pipe seals are sold separately.

Does this kit fit 2016 and older Duramax trucks?
No. See the 2001-2010 Thermostat Kit or the 2011-2016 LML Thermostat Kit.

For warranty information, see our Warranty Policy.