Cummins ISC 8.3L: Widely used in medium-duty delivery trucks, fire trucks, and school buses.

Cummins ISL 9L: Common in refuse (garbage) trucks, vocational vehicles, and motorhomes.

Paccar PX8: Found in Kenworth and Peterbilt medium-duty trucks (this is the Paccar-branded version of the Cummins ISC).

Function: It acts as a chemical reactor. When hot exhaust gases pass through the precious metal-coated honeycomb structure inside, a catalytic reaction occurs.

Pollution Reduction: It converts carbon monoxide (CO) and unburnt hydrocarbons (HC) into less harmful carbon dioxide and water vapor.

Heat Generation: It also helps manage heat for the DPF (Diesel Particulate Filter). By oxidizing fuel in the exhaust stream, it raises temperatures enough to burn off trapped soot (a process called regeneration).

Active Regeneration Issues: The most common sign is the engine's inability to complete a DPF regeneration. If the catalyst cannot generate enough heat to burn off soot, your DPF will clog frequently.

Reduced Engine Power: You may experience a significant loss of horsepower or find the vehicle is stuck in "Limp Mode," which limits speed and RPM to prevent further engine damage.

Increased Fuel Consumption: A restricted or inefficient catalyst forces the engine to work harder and use more fuel during failed regeneration attempts.

Fault Codes (CEL): You will likely see a Check Engine Light or "Aftertreatment" warning. Common Cummins fault codes include FC1691 (Catalyst Efficiency Low) or FC2639 (Manual Regeneration Required).

Strong Exhaust Odor: A failing DOC may allow unburnt hydrocarbons to pass through, resulting in a heavy diesel smell or even visible white smoke from the tailpipe.