Dorman Front Coolant Reservoir 603-126 — Non‑Pressurized Replacement with Cap for Select GM SUVs (2002–2009)

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

This replacement coolant reservoir is engineered to match the fit and function of the factory part while resisting the heat and vibration encountered under the hood. Built from durable materials and supplied with a matching cap, the Dorman 603-126 is intended for straightforward installation on compatible vehicles including Buick Rainier, Chevrolet Trailblazer and Trailblazer EXT, GMC Envoy and Envoy XL/XUV, Isuzu Ascender, Oldsmobile Bravada and Saab 9-7x across the model years listed in the product fitment tool. Before ordering, confirm exact model and trim fit using your vehicle details in the garage tool.

Will this reservoir fit my vehicle and how do I confirm compatibility?

This reservoir is made to be a direct replacement for specific GM-derived SUVs across the 2002–2009 model years shown in the fitment list. Fitment depends on model year and trim, so verification is important to avoid returns.

• Compatible makes include Buick, Chevrolet, GMC, Isuzu, Oldsmobile and Saab (see fitment list).
• Use the garage tool on the product page to enter make, model and trim.
• Part number: Dorman 603-126; cross-check with your old reservoir before purchase.

How does installation compare to the original part?

Dorman manufactures this reservoir to match the original geometry and mounting locations so installation follows the same steps as the factory tank. Basic hand tools are typically all that’s required, and the included cap eliminates the need to source additional components.

• Direct replacement design aligns with factory mounting points and hose connections.
• Expect a simple swap using common tools—no modifications should be necessary.
• Includes a compatible cap to restore the system’s sealing and overflow control.

What materials and durability features does this reservoir offer?

The reservoir is molded from materials selected for underhood durability, resisting heat and vibration encountered during normal vehicle operation. Dorman applies decades of aftermarket experience to its designs to improve longevity over worn or brittle original tanks.

• Manufactured from heat- and vibration-resistant polymer for long service life.
• Reinforced mounting areas to prevent cracking at attachment points.
• Engineered to handle typical coolant exposure without rapid degradation.

What should I inspect when replacing my coolant reservoir?

When installing a new tank, check associated hoses, clamps and the cap for wear—these components affect system performance and may require replacement at the same time. Inspect coolant level sensor ports and grommets to ensure a proper seal and electrical connection if applicable.

• Examine hose ends and replace brittle or swollen hoses and old clamps.
• Verify the cap seats correctly and seals the reservoir neck.
• Confirm any level sensor or mounting grommet is intact and properly seated.

Who is this for: The Dorman 603-126 is aimed at owners and technicians restoring cooling system function on compatible mid‑2000s GM-based SUVs who want a reliable, plug-and-play replacement tank. It’s appropriate for DIY mechanics and professional shops that need a durable, OEM‑style reservoir and cap to complete coolant system repairs or maintenance without sourcing aftermarket adapters or modifying the vehicle.