Dorman Front Non-Pressurized Coolant Reservoir with Cap — Direct-Fit Molded Plastic for Select GM SUVs

Dorman 603-126 is a direct-fit front engine coolant reservoir with cap designed to replace the original expansion tank on select GM and related SUVs.

This replacement coolant reservoir is built to restore the cooling-system fluid capacity and appearance of your vehicle without modifying mounting points or hoses. Molded from resilient materials to resist underhood heat and vibration, the tank and cap are manufactured to match the shape and connection layout of the factory part. Use your vehicle make, model and trim in the garage tool to confirm compatibility before ordering.

Which vehicles does this reservoir fit?

The Dorman 603-126 fits a range of GM-based SUVs and related models produced in the early-to-late 2000s. It covers multiple model years across Buick, Chevrolet, GMC, Isuzu, Oldsmobile and Saab lineups that share the same cooling system layout. Always verify fitment with your specific trim and production year.

• Buick Rainier: 2004–2007
• Chevrolet Trailblazer & Trailblazer EXT: 2002–2009 (select years)
• GMC Envoy/Envoy XL/XUV: 2002–2006 (and select later years)
• Isuzu Ascender, Oldsmobile Bravada, Saab 9-7x: various 2002–2009 years

How is this part built to hold up under the hood?

Dorman designs replacement reservoirs with materials selected for thermal stability and vibration resistance so the part endures normal engine bay conditions. The tank resists warping and cracking from heat cycling, and the cap seals to maintain coolant containment at the intended pressure range for non-pressurized systems. Manufacturing tolerances match original mounting and hose locations to prevent stress at connection points.

• Durable molded plastic resists heat-related degradation
• Cap engineered for proper non-pressurized sealing
• Reinforced connection points reduce leak risk
• Factory-style mounting and port placement for a snug fit

What should I expect during installation?

Installation is straightforward for anyone comfortable with basic underhood work: drain or capture excess coolant, remove retained clips and hoses from the old tank, swap the reservoir and reconnect hoses and the cap. The Dorman unit matches the original part's openings and mounting brackets to minimize trimming or extra hardware. If you prefer, a professional shop can complete the replacement within an hour on most compatible vehicles.

• No modifications required for correct-fit trims
• Standard hand tools typically suffice for removal and installation
• Replace coolant if contaminated or if system was open to air
• Double-check hose clamps and cap seating for leaks after refill

Why choose an aftermarket replacement from Dorman?

Dorman blends practical design with decades of aftermarket experience to offer cost-effective, reliable replacement components. This reservoir is engineered to mirror the original part’s fit and function while using materials and production controls intended for long-term service. Choosing a reputable aftermarket option can restore system operation quickly and at lower cost than some OEM alternatives.

• Engineered for interchangeability with original equipment layouts
• Manufactured to withstand standard underhood stresses
• Backed by Dorman’s long-standing aftermarket expertise
• Often available at a lower price than equivalent OEM parts

Who is this for: The Dorman 603-126 reservoir is suited to DIYers and professional technicians repairing cooling-system components on compatible Buick, Chevrolet, GMC, Isuzu, Oldsmobile and Saab SUVs from the 2002–2009 model years. Choose this part if you need a direct replacement that restores original mounting locations and hose connections with durable materials and a ready-to-install cap. Confirm vehicle fitment with the garage tool before purchase to ensure proper compatibility.