DOC vs DPF vs SCR — Which Emissions System Does Your Machine Have?

Not every “catalytic converter” on a Tier 4 machine is the same part — and only some of them are a rebuildable core. Here’s how to tell what you have before you ship us anything.

DOC — diesel oxidation catalyst (we rebuild this)

The DOC is a flow-through honeycomb catalyst that oxidizes carbon monoxide and hydrocarbons. It’s the classic rebuildable core — we replace the internals and send it back. Nearly every Tier 4 diesel under ~74 hp has one (chippers, skid steers, mini excavators, mulchers, compact tractors, booms), either standalone or ahead of a DPF.

DPF — diesel particulate filter (cleaned, not rebuilt)

The DPF is a wall-flow filter that traps soot and is periodically regenerated. It sits right next to the DOC and is often in the same canister, but it’s cleaned or replaced, not rebuilt. If your machine has a DOC+DPF, we rebuild the DOC portion.

SCR / DEF — selective catalytic reduction (not a rebuild)

SCR uses diesel exhaust fluid (DEF) and a specialized catalyst to break down NOx. If your machine takes DEF, that catalyst isn’t a rebuildable oxidation core. Most row-crop tractors, combines, sprayers, and on-highway truck chassis are SCR/DEF — a different repair path than what we offer.

3-way catalyst — gas & LP engines (we rebuild this)

Gas and propane engines (Ford, GM, PSI, Kubota WG) use a 3-way catalytic converter. These are rebuildable cores too.

The 30-second check

Does your machine take DEF? If no, it almost certainly has a rebuildable DOC (or a 3-way cat on gas/LP). If yes, it’s an SCR system we don’t rebuild. Still unsure? Call 774-701-6374 with your engine model and serial and we’ll tell you exactly what you have.

Pricing: Off-road converter rebuilds are quoted per engine and converter — call 774-701-6374 for your price.

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See our catalytic converter rebuild program for the full list of chippers and off-road equipment we rebuild.