X-TRA Cool L5P Transmission Cooler Thermostatic Bypass XD396 XDP


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XDP Xtreme Diesel Performance SKU: XD396

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The XDP X-TRA Cool L5P Transmission Cooler Thermostatic Bypass XD396 helps lower temperatures and prolong the life of the Allison transmission in your 2017-2019 GM 6.6L Duramax L5P. From the factory, the L5P Duramax trucks are equipped with a transmission cooler bypass valve.

The factory installed thermostatic valve is designed to provide quicker warm-ups in cold weather. While this is a great idea in theory, failures in the field are proving that this factory design is unreliable and causes excessive heat in your transmission. The issue with the factory bypass valve is that the design restricts flow, and when the thermostat fails, it causes the fluid to bypass the transmission cooler completely.

This results in elevated temperatures, premature transmission wear, and in some cases can lead to early transmission failure. Once line pressure is increased with common performance upgrades, the issue becomes compounded from the added pressure and heat.

The XDP XD396 Thermostat Bypass is a completely re-designed bypass block made from 6061 billet aluminum. This bypass valve removes the factory thermostat and allows for unrestricted flow to the transmission cooler at all times. This unrestricted flow can provide up to a 50 degree drop in transmission fluid temperature!

This results in better cooling, less wear and tear, and in many cases can help extend transmission life. This valve also allows for a transmission fluid flush without requiring a hot flush machine as there is no thermostat required to open.


  • Direct-Fit
  • Replacement No Cutting Of Factory Lines
  • Easily Installs With Hand Tools
  • Helps Improves Transmission Life
  • Eliminates Hot Fluid Recirculation
  • Lowers Transmission Fluid Temperature Up To 50 Degrees