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Steve Hoke

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With all the focus on DPFs, the DOC is often forgotten as a part of your aftertreatment system. The DOC is the first line of defense against harmful chemicals in diesel engine exhaust, and it should be cared for and maintained just like the rest of your truck's emissions system. Today, we're taking a deep dive into what a DOC is, how it works, and the maintenance you need to do to keep your truck on the road. 

What is a DOC? 

The DOC or sometimes referred to as the diesel catalytic converter, is part of your truck's emissions system. It usually has a stainless-steel exterior housing which contains a honeycomb structure with a catalytic coating on a cordierite or metallic substrate for oxidizing exhaust. It operates in a passive-only mode that requires no active regeneration or special duty cycle requirements. The honeycomb structure gives more surface area that engine exhaust passes through. This hot exhaust reacts with the precious metal coating causing a catalytic reaction, breaking down the pollutants in the exhaust into less harmful components.  

Diesel emissions systems on vehicles with diesel engines are regulated in the United States by the Environmental Protection Agency or EPA. The California Air Resources Board, CARB for short, works with the EPA to evaluate how effective these diesel oxidation catalysts are.

Where is a DOC located?  

The location of the DOC depends on the vehicle manufacturer and the year of the vehicle. But the DOC is usually just before the diesel particulate filter DPF in the flow of the aftertreatment system.  On 2007 through to 2009 engines, the system typically consists of a DPF and DOC. On 2010 and newer engines, the system consists of a DPF DOC, and SCR; the presence of a DEF tank easily identifies these.

What kinds of vehicles have a DOC? 

There are many applications for a diesel oxidation catalyst. DOCs can be found on off-highway equipment (tier 4 interim or tier 4 final); used in mining, logging, industrial, and construction applications. You can also find DOCs on marine vessels with diesel engines and stationary equipment like large diesel-powered generators.  

All on-highway heavy-duty diesel applications with 2007 or newer engines will be equipped with a DOC. These include school buses, transit buses, long-haul semi-trucks, inner-city delivery vehicles, garbage trucks, utility vehicles, and many other on-road applications.

What does a DOC do? 

Untreated diesel engine exhaust contains many harmful chemicals, including carbon monoxide, hydrocarbons, and organic particulates. The DOC passively converts these into carbon dioxide and water by adding oxygen (the "oxidation" part of the name). It is one in a series of steps that reduce pollutants and particulates before your truck's exhaust is released into the environment. For example, hydrocarbons may be reduced by 40 to 75 percent and carbon monoxide by 10 to 60 percent.

How does a DOC work?  

The diesel oxidation catalyst (DOC) gets its name from the way it oxidizes exhaust gas components. Fundamentally, oxidation is the loss of electrons. So, when a compound loses one or more electrons, it is considered to be oxidized. Generally speaking, metals are easily oxidized, which is why the DOC uses precious metals as the catalyst for the oxidation process.  

After exiting the combustion chamber, exhaust flows through the DOC. As harmful chemicals like carbon monoxide (CO), hydrocarbons (HC), and the organic fraction of diesel particulates (OF), as well as other non-regulated emissions, pass over the reactive precious metals in the structure, they bond with oxygen molecules, turning into much less harmful molecules. The exhaust then flows to the DPF, where more chemicals and particulate matter wait for a regeneration cycle that burns them into ash.  

Vehicles manufactured after 2010 have diesel particulate filters (DPFs) and a selective catalytic reduction (SCR) which need nitrogen dioxide (NO2) to perform properly and reduce nitrogen oxides (NOx) which is part of the family of air-polluting chemical compounds. The DOC will oxidize nitric oxide (NO), also called nitrogen monoxide, into nitrogen dioxide (NO2). 

The whole process is broken down into three steps: 
  • The precious metals bond with oxygen molecules. 
  • Carbon monoxide and other hydrocarbons react to the oxidized precious metals. 
  • The reaction creates CO2 and water vapor which is much less harmful to the environment.

The result is that the diesel engine's exhaust pollutes the air far less than ever before, and new targets set for 2027 will further reduce the air pollutants in diesel exhaust.

Why does the DOC fail? 

Over time, the precious metals in the DOC will naturally deteriorate, meaning you will have to replace them after about ten years or so. However, there can be many causes of why your DOC could fail prematurely. For example, if there are issues upstream, like exhaust leaks, or elsewhere in the aftertreatment system, the DOC could experience buildup or contamination, causing it to fail.

Other DOC failures can come from:  
  • Oil contamination. This may be detected by blue smoke coming from your emission system. Also, be aware of adding oil to your engine. This can also be an indicator of oil going through your system.   
  • Fuel contamination. Signs of fuel issues can include white smoke or black smoke coming out of the exhaust pipe. 
  • Coolant contamination. A visual sign may be white smoke coming out of the exhaust pipe. Be aware of adding coolant to your engine as well. Ethylene glycol will poison the catalyst almost immediately. 
  • Excessive idling. Excessive idling is one of the harshest environments for the aftertreatment system and can create face plugging. 
  • Exhaust leaks. Today's emission systems are built to deliver heat and pressure from the turbo through to the tailpipe. Any leaks will allow heat and pressure to escape, potentially compromising the efficiency of the entire system. 
  • Lack of maintenance. An aftertreatment system is an exhaust filter, and, like any filter, they need to be maintained. System maintenance should be tailored to the duty cycle. Harsh duty cycles should be serviced more frequently. Harsh duty cycles include school buses, curb site trash collection vehicles, inner-city delivery vehicles, etc.
Can the DOC be cleaned? 

Yes! Anytime your DPF is removed for routine cleaning, the DOC should be removed, inspected, and cleaned simultaneously with the same cleaning method. Because the DOC is a flow-through device, what is in the DPF has passed through the DOC, and it should be cleaned as well. If there is buildup on the DOC, it can become face plugged. Therefore, please remember to install new gaskets and clamps when reinstalling any time you service your system. This will help ensure your system will function properly.

What is face plugging in a DOC? 

Face plugging is an extreme buildup of carbon on the inlet side of your DOC. This blockage causes exhaust back pressure and restricts the exhaust flow, which hinders the performance of your entire aftertreatment system, including the DPF and SCR. Face plugging can have several causes. For example, trucks that spend a lot of time idling or hauling light workloads are more at risk of face plugging. Also, if there are problems upstream with your engine, contaminants could clog up your DOC. 

Important Takeaways 
  • Face plugging severely hinders the performance of the DPF & SCR downstream 
  • A face plugged DOC is costly and should be prevented with regular maintenance of the diesel emissions system. 
  • Don't rely on performing forced DPF regenerations to regularly burn the soot out of your DPF and in turn, reduce the chance of face plugging the DOC. Instead, have your DPF and DOC regularly cleaned at an Aftertreatment Cleaning Facility like Diesel Emissions Service.  
Fixing a DOC that has been face plugged? 

The best way to fix this situation is to take the DPF, DOC to a professional aftertreatment cleaning facility with the necessary equipment to clean both the DOC, DPF. If you remove the parts and reinstall them yourself after being cleaned, you can save some money on labor. Don't forget to use brand new clamps and gaskets when reinstalling these aftertreatment parts.

Preventative Maintenance is Very Important 

Face plugging the DOC is more likely to be a problem for trucks that spend most of their time on inner-city roads at lower speeds or are not loaded to their maximum capacity. The engine and exhaust system do not get hot enough for passive regeneration, and these trucks often require forced regeneration instead. 

Conversely, trucks that operate at highway speeds for sustained periods are loaded to their maximum capacity, and travel on mountain roads generate sufficient exhaust temperatures so that the issue of face plugging is far less common.  

Every truck operates in slightly different environments and can even be affected by differences in the style of driving of the operator. So, it is important to establish a preventative maintenance program tailored for each truck's specifics. This should include regular cleaning of the DOC and diesel particulate filter DPF and inspections for upstream issues. 

Tools that You Should Have with You 

No matter how diligent you are with your maintenance plan, you may still run into issues on the road with your DOC or other aftertreatment systems parts. Having an on-board tool to identify what a fault code is indicating is very useful.  

We recommend that every truck should have a Diesel Decoder on board with them. This device will pay for itself in one saved tow or dealer diagnostic visit. It is designed for heavy-duty truck owner-operators and small commercial fleets and is a powerful one-of-a-kind diagnostic tool for all heavy-duty truck makes. 

Learn more about Diesel Decoder here.

When should a DOC be replaced? 

Because DOCs work passively in your aftertreatment system and have no moving parts, they can usually last ten years or more. Eventually, the precious metals in your DOC will deteriorate, and you'll need to get it replaced. If your DOC is causing problems prematurely, there may be other factors, such as face plugging or a lack of regular maintenance. 

Oil, fuel, and coolant exposure will cause damage to the DOC catalyst. Therefore, it is very important to understand why the original unit failed and be certain underlying causes have been addressed. Engine issues will contribute to DOC failure and may be accompanied by other symptoms such as plugged EGR valves. 

How much does a DOC usually cost? 

The cost of your DOC replacement depends on the year, make, and model of your truck's engine. On average, heavy-duty replacement DOCs can range from $1,500 to upwards of $3,000. These prices might seem high, but remember that DOCs contain precious metals like platinum, which drives up the price. Taking care of your DOC and the entire aftertreatment system is a smart way to save money.  

Be wary of low-cost replacements. The DOC is the heart of your emission system, reputable US manufacturers are EPA self-certified, and they develop products engineered to work with the existing system components.  In vehicles newer than 2010, the DOC, DPF, and SCR are created to work together. If you change the performance of one component, with, for example, a low-quality replacement that doesn't perform at OEM specifications, you will compromise the entire system.  

A word of caution, there are several options for replacement units, new OEM, reman units, and new aftermarket units. Generally, the new OEM and reman units come with a core requirement. If the unit you are exchanging is cracked or otherwise damaged, the price may exceed the cost of a new aftermarket unit as they generally have no core requirement.

How can I tell if a DOC replacement is high-quality or low-quality? 

Your DOC replacement should be an "OEM replacement," sometimes called "exact fit." These terms mean that the replacement was precisely engineered for your specific make and model engine. The DOC replacement should also come with a warranty, and the company you purchase from should have a great reputation. It's always helpful if your supplier has an expert hotline to make sure you're buying the right part for your truck.    

A word on reman units: Since a reman unit is just someone's DOC that has been exchanged, cleaned, and then made available for sale, there is no way to know the life cycle of the unit.  

You can't know how many years it has been in service, what the unit has been exposed to, and what the previous duty cycle was like. Your truck may be only 4-years old, but the reman DOC you are purchasing may be from a truck that was 8-years old with a lot more miles than yours. You purchase the reman unit thinking you are saving money, and the DOC completely fails only a few weeks later and needs to be replaced. Meanwhile, your DOC unit has been entered into the exchange program, and someone else will now have it on their truck and get several years of use out of it.

Where should I buy a replacement DOC? 

There are many options available, including buying from the dealer or your local aftermarket parts store. The problem with those options is the dealers are very expensive, and the aftermarket parts stores do not always have the selection or the expertise to help you if you have a problem.  

DPFPartsDirect.com is the best place to buy your replacement diesel particulate filter and any other diesel emissions system-related parts.

  • It is easy to buy from us online.  
  • We have a huge inventory in stock and ready to be shipped.  
  • We sell high-quality OEM and aftermarket replacement parts.  
  • Orders typically ship the same day.  
  • Shipping costs are often included in the price of the part.  
  • We have diesel technicians available to provide you with assistance if you need it.

Buy all your diesel emissions systems replacement parts from DPFPartsDirect.com today.


DOC – Diesel Oxidation Catalyst 

DPF – Diesel Particulate Filter 

SCR – Selective Catalytic Reduction 

DEF – Diesel Exhaust Fluid 

OEM – Original Equipment Manufacturer