Dorman Front Pressurized Coolant Reservoir with Cap for Chevrolet/GMC 2500/3500 (2001–2007) — Direct-Fit OEM-Style

Dorman 603-125 is a direct-fit, pressurized front coolant reservoir with cap engineered to replace the original overflow tank on select Chevrolet and GMC 2500/3500 models.

This replacement coolant reservoir is built to restore the cooling-system overflow function for late-model Chevy and GMC heavy-duty trucks. Constructed from heat- and vibration-tolerant materials, it is manufactured to align with the original mounting points and hose connections so installation is straightforward. Confirm compatibility with your exact year, model, and trim using the vehicle fitment tool before purchase.

How does this reservoir match the original part?

The Dorman 603-125 is designed to mirror the fit and functional layout of the factory tank, including inlet/outlet locations and mounting tabs. That means fewer modifications and a faster, cleaner install compared with universal tanks. It also comes with a pressure cap manufactured to maintain the proper system pressures for compatible applications.

• OEM-style mounting points and hose ports for direct replacement
• Integrated pressurized cap compatible with specified Chevy/GMC models
• Preformed shapes to match factory fitment and clearance

Will this hold up under underhood heat and vibration?

The reservoir is produced from materials selected for automotive underhood environments to resist heat cycling and mechanical stress. Dorman applies decades of aftermarket experience to material choice and molding processes so the tank resists premature cracking or deformation. Still, proper installation and using the correct cap and hoses are important to long-term durability.

• Durable thermoplastic construction formulated for engine compartments
• Reinforced mounting areas to reduce stress points
• Factory-style cap designed for the pressure range of specified vehicles

Which vehicles does this reservoir fit and how do I verify fitment?

This reservoir fits select Chevrolet Silverado and GMC Sierra 2500/3500 series trucks from 2001–2007, including Classic model years indicated by the manufacturer. Because trim and submodel differences can affect fit, use the site's garage or fitment tool to enter your exact year, make, and trim before ordering. If your vehicle isn't listed, compare mounting locations and hose sizes to confirm compatibility.

• Compatible with Chevrolet Silverado 2500/3500 and Classic variants (2001–2007 where specified)
• Compatible with GMC Sierra 2500/3500 and Classic variants (2001–2007 where specified)
• Verify fitment using the vehicle lookup/garage tool prior to purchase

How easy is installation for a DIY mechanic?

Installation is intended to be straightforward for someone comfortable with basic cooling-system service—typically involving draining minimal coolant, disconnecting hoses, removing a few fasteners, and swapping the tank and cap. Because it mirrors the factory layout, special modifications or cutting are rarely required. Always allow the engine to cool and follow safety procedures when servicing the cooling system.

• Direct-replacement design reduces the need for drilling or trimming
• Standard fasteners and hose clamps are used for attachment
• Recommended: cool engine, proper disposal of any drained coolant

Who is this for: The Dorman 603-125 reservoir is aimed at owners and technicians working on Chevrolet Silverado and GMC Sierra heavy-duty trucks from the early-2000s who need a reliable, OEM-compatible replacement for a cracked, leaking, or missing coolant overflow tank. It’s suitable for DIYers who want a straightforward swap and for shops seeking a ready-to-install part that restores the cooling system’s pressurized overflow function without fabricating mounts or hose adapters.