Since 2007, the majority of vehicles with a diesel engine have a catalytic converter.

The benefits of these converters are that they turn 90% of harmful hydrocarbon and carbon monoxide emissions into less harmful gasses.

Essentially, a catalytic converter helps reduce air pollution. 

But do diesel trucks have catalytic converters? 

These days, depending on year of make, all diesel engine trucks manufactured from 2007 onwards are equipped with catalytic converters. So not all trucks on the roads will have a catalytic converter to help improve the engine’s performance and to help reduce air pollutants.  

In this comprehensive guide, you’ll learn how diesel catalytic converters work, the symptoms of a failing converter, the cost of these converters, and how long they last. 

What Is a Diesel Catalytic Converter? 

A diesel catalytic converter is a catalytic converter (or cat converter) that’s made specifically for diesel engines. 

A diesel engine and gasoline engine are fundamentally different, so they each need to have their own engine-specific converters.    

Engines that work with gasoline (aka internal combustion engine) need a spark to ignite the fuel, while diesel engines use compression.

Since diesel fuel is lean burning, it has a high stoichiometric air (air to fuel) ratio to ensure the oxygen present is sufficient for total ignition. 

As such, a diesel engine needs a different catalytic converter than a high-octane gas-powered engine.  

A catalytic converter is a device that controls exhaust emissions to convert toxic hydrocarbon and carbon monoxide gasses and other pollutants into less-toxic pollutants.

What Are the Benefits of Diesel Catalytic Converters? 

Diesel catalytic converters:

  • Reduce airborne pollutants that are harmful to the environment and humans 
  • Improve your vehicle’s engine efficiency  

How Do Diesel Catalytic Converters Work?

How Do Diesel Catalytic Converters Work

In the 1990s, diesels started using catalytic converters.

The converter is usually located between the engine and the muffler. It’s fitted as a part of the exhaust system. 

Inside the metal casing of the diesel catalytic converter, two ceramic blocks contain thousands of micro-cellular units, which remind you of a honeycomb. 

Both of the ceramic blocks are coated in metal – platinum or palladium.

Because of these valuable metals that aren’t likely to corrode, thieves like to steal these converters to sell it for their scrap value.   

So how does a cat converter for a diesel engine work?  

A catalytic converter is a pollution filter and a rearranger.

The unit rearranges atoms and this transforms or catalyzes chemicals into exhaust fumes. 

The atoms need to go through a few phases to complete the process whereby they become fumes: 

  1. Exhaust gas recirculation (EGR) 
  2. Diesel particulate filtration (DPF) 
  3. Diesel oxidation catalyst (DOC) 
  4. Selective catalytic reduction (SCR) 

During the DOC and SCR stages, the diesel catalytic converter produces chemical reactions so emissions are decreased. 

Most engines have a DPF, DOC, and SCR, so we’ll look at these processes in more detail.

Diesel Oxidation Catalyst

Every diesel cat converter has a diesel oxidation catalyst.

This unit converts carbon monoxide and hydrocarbon into carbon dioxide and water so volatile hydrocarbons and other byproducts are catalyzed into less harmful gasses. 

It also decomposes any unspent fuel. 

When you consider the function or purpose of the DOC, it’s the same for diesel and gasoline catalytic converters.

However, a diesel catalytic converter doesn’t have rhodium like a gas cat converter has.

The DOC is typically the smallest part of the cat converter and the first part of the exhaust system; however, it has the highest recycle value.

It’s in the DOC where most of the palladium and platinum is located.

Diesel Particulate Filter

Another difference between gas engines and diesel engines is that the latter produce an extra pollutant: soot, or particulates. 

As such, the diesel particulate filter in a diesel engine catalytic converter captures these pollutants.

The DPF uses a wall-flow monolith that has alternating open and closed channels. 

The particulates enter the converter from an inlet, and then they are held in the DPF until they are broken down and removed in a regeneration process.

The regeneration process is somewhat similar to the DOC’s oxidation process where harmful chemicals are burned to create water and carbon dioxide.

The DPF has lots of small amounts of palladium and platinum, which aids the catalytic reaction process. 

The DPF also needs to be heated so the soot can burn and pass through the filter walls.

From there, the soot leaves via the tailpipe.

Selective Catalytic Reduction

The selective catalytic reduction part of the cat converter reduced nitrogen oxide gasses. 

In a three-way cat converter (for gasoline-powered engines), rhodium catalyzes the reduction reaction to convert nitrogen oxide into oxygen and nitrogen. 

But since a diesel engine has a high oxygen content, the converter needs a SCR. 

The selective catalytic reduction unit uses diesel exhaust fluid (DEF), an additive that comprises either urea or ammonia to remove the nitrogen oxide from the exhaust.

This is an open-loop system that reduces nitrogen oxide emissions by 75% to 90%. 

The SCR also helps reduce hydrocarbon emissions, but it only does this by 8%. 

When you use low sulfur fuel, the performance of the SCR is improved.  

Some SCRs have an ammonia slip catalyst, which is located near the back of the biscuit.

This catalyst has a small amount of platinum to burn off excess ammonia from the diesel exhaust fluid. 

How Long Do Diesel Catalytic Converters Last?

A diesel catalytic converter typically lasts around a decade (10 years) or 70,000 to 100,000 miles. More technically, how long the diesel cat lasts depends on how well your vehicle is maintained and the length of your trips. 

Regular short trips wear the cat out prematurely. 


A catalytic converter properly functions at a high temperature 752 °F (400 °C). If you drive a short distance, the engine is only switched on for a short period.

The consequence is that the converter doesn’t warm up enough to the appropriate operating temperature needed to catalyze the chemicals. 

The more often you drive for a short distance or make lots of stops, the quicker the catalytic converter wears out.

What Causes a Catalytic Converter to Fail? 

There are various reasons why a catalytic converter can fail. If you notice that your diesel cat is failing, you can act and get it replaced. 

A premature failing can result because of: 

  • A carbon deposit buildup 
  • Physical damage 
  • Poor engine maintenance 
  • Restriction of the free flow of exhaust gasses 
  • Overheating  

Most trucks are fitted with a few oxygen sensors.

These help determine if there’s something wrong with the diesel catalytic converter. 

If the sensor doesn’t get a proper reading, your Check Engine Light may come on. Use a scanning tool to see what’s causing the issue. 

There are also other symptoms that indicate the cat is failing: 

  • Engine isn’t performing optimally 
  • You can’t accelerate as well as you normally do 
  • There’s a rotten egg smell from the truck’s exhaust 
  • The exhaust smoke is dark

How Much Does a Diesel Catalytic Converter Cost?

The cost of a catalytic converter is quite expensive, but not having one isn’t safe or legal. 

To replace the catalytic converter for a diesel-powered engine will cost you between $950 and $2,500 (or more). 

Theft-proof devices to prevent thieves stealing your diesel catalytic converter are a must.

Consider installing a shield to protect your cat.

These shields are often made from high-strength aluminum to provide protection without adding unnecessary weight to your truck. 

Brands you could consider are: 

My Last Thoughts on Catalytic Converters for Trucks 

A catalytic converter for your diesel truck is non-negotiable.

It helps reduce harmful pollutants that have an adverse effect on climate change and global warming. 

It’s essential to know what symptoms to look for when your diesel cat is starting to fail so you can act quickly and get it replaced as soon as possible.  

Since there’s evidence that thieves are targeting trucks and other diesel vehicles, it’s in your best interest to ensure you protect your catalytic converter.