In 2011, an investigation conducted by the Washington State Fire Department linked a truck's DPF failure to one of the biggest fires in the state. Over 3,600 acres of land burned, destroying over 100 buildings, including homes. While the cause of the DPF's demise wasn't determined, all too often these kinds of dangerous failures start with—or are exacerbated by—serious diesel oxidation catalyst problems. 

Don’t let that kind of breakdown happen to you. Instead, learn what causes DOC failure, and take steps to prevent it. Or, at least, mitigate the worst of its effects. 

What Is The Diesel Oxidation Catalyst? 

A diesel oxidation catalyst (DOC) is similar to the catalytic converter in a car's engine. It cuts down on harmful emissions by transforming toxic particles in diesel engine exhaust through chemical reactions. It also increases the concentration of NO2 in the diesel particulate filter to facilitate DPF regeneration. 

How Does A Diesel Oxidation Catalyst Work?

Coated with rare metals, DOCs scaffold and enact a chemical reaction. They convert carbon monoxide (CO), hydrocarbons (HC), and other particulate pollutants in exhaust (diesel engine waste) into carbon dioxide (CO₂), water vapor, and nitrogen dioxide (NO₂)—all safer outputs.

Signs An Engine’s DOC Might Be Having Trouble 

A warning light should turn on if a truck’s DOC is losing functionality. But, other signs might grant an earlier warning. Look for:

• Reduced fuel efficiency

• An increase in exhaust smoke volume (particularly white smoke)

• Frequent DPF regeneration cycles

• Increased engine heat

5 Most Common Causes of Diesel Oxidation Catalyst Failure 

DOC failure requires swift attention. Fortunately, not every cause is equally challenging to resolve. When a DOC malfunctions, there are five usual suspects. 

1. Clogging Due To Soot Buildup

“Soot” is the informal term for the solid particulates and hydrocarbons the DOC chemically converts. But, even though it runs continuously, the device cannot always convert every particle. Soot can collect in it, clogging it over time. If soot fills the DOC’s pores or substrate, it can harden, impairing the device’s function. 

2. Rare Metal Degradation Within The Catalyst

DOCs typically incorporate coatings of rare metals (often platinum and palladium) known for their high catalytic properties. These elements accelerate the oxidation process, and they enable oxidation at a lower temperature than would otherwise be necessary. 

However, substances found in certain diesel fuels, sulfur compounds, and oil additives can cause the metals to degrade. Ultra-high exhaust temperatures can also break down the rare metals, as the heat can sinter the metal particles. 

3. Low Exhaust Temperatures

While too-high temperatures can damage DOCs, too-low temps also cause problems. Though the DOC is meant to run continuously while the truck runs, it also has a “light off” temperature. When exhaust dips below that temperature threshold, it turns off, unable to oxidate the pollutants. 

Exhaust temperatures can drop if:

• The exhaust system lacks effective insulation against cold climates

• The fuel injector timing is off, allowing too much cooling

• There are leaks or excessive back pressure

• The exhaust temperature sensor is dysfunctional or broken 

4. Exhaust Leaks

Exhaust leaks are tears, holes, or gaps within the exhaust system parts that let exhaust waste and fumes escape the directed course, disrupting the flow of the system’s gasses. Depending on location, leaks can let hot exhaust gas and hydrocarbons bypass the DOC.

The unplanned outflow of hot gas lowers the temperature of the exhaust remaining in the system. As a result, the DOC may not be able to oxidate the exhaust, leading to dangerous levels of soot buildup. 

Moreover, the change in pressure due to airflow shifts increases stress on the DOC. This, too, can exacerbate wear and deterioration.

5. Problems In Other Engine Parts Affecting The DOC

The different, interconnected systems of an engine are in an enclosed space. Misaligned, damaged, clogged, or otherwise poorly functioning parts along the whole exhaust chain often impact the DOC. The most common culprits are:

• Turbochargers

• Fuel injectors

• Coolant system parts

• ERG valves

Potential Fixes To Diesel Oxidation Catalyst Problems 

To fix a damaged DOC, or to prevent it from getting bad, there are a few useful strategies to try. These include:

• Regular monitoring

• Cleaning

• Component repair or replacement

Monitoring

Monitoring can be done at regular intervals using comprehensive digital systems. Drivers can also integrate an NO₂ monitor, as a sudden increase or drop in NO2 production can indicate a problem.

Cleaning

Cleaning the DOC is best done at an automotive servicer with a specialized thermal oven designed to bake soot off exhaust system parts. 

That said, it’s also possible to use highly pressurized air to blast clean a DOC, or utilize precisely formulated liquid solvents. No matter which route you take, though, working with an automotive professional is the only way to safely clean these engine parts.

Repair and Replacement

Upon inspection, a DOC may need to be repaired or replaced. A replacement requires compatibility with the exhaust system at hand, and necessary specifications are typically listed in the truck’s owner’s manual. 

Drive From DOC Failure To Long-Term Success: Point Spring Gets You There

If your exhaust system is in trouble, don’t delay any longer. Instead, connect with the experts. 

Our professional, certified technicians specialize in solving the particular problems faced by heavy-duty vehicle drivers. No matter what your diesel engine dilemma is, we've got it covered. For an appointment or consultation, call us at 1 (412) 264-6714.