Anyone that has been in the market for a heavy duty truck since 2010 has probably heard a lot about emissions systems. The fact is, even gas engines have similar systems, but with diesels they seem to be less reliable and cost more. Although anyone that has had their catalytic converter stolen from their gasoline powered car in the last couple years can commiserate with how expensive the emissions system can be to fix.

Exhaust Gas Recirculation (EGR)

This exists even on gasoline cars, broadly speaking, its circulating exhaust gas back into your intake to reduce the amount of oxygen in the combustion chamber to reduce nitrogen dioxide (NO2 or smog) emissions. Diesel engines use a lot more air so the exhaust recirculation system is more extensive and elaborate. Valves and controls for such gas with so much heat and pressure are inherently expensive and prone to failure.

Selective Catalytic Reduction (SCR)

SCR is the system that uses the DEF fluid that you need to fill up on your diesel vehicle truck periodically. A catalyst is a substance that accelerates a chemical reaction. DEF fluid is primarily urea which converts the nitrogen dioxide to carbon dioxide and nitrogen. SCR was invented in the 1960’s but became widespread in the US after the 2010 EPA regulations went into effect.

Diesel Particulate Filter (DPF)

A diesel particulate filter is pretty much what it sounds like, a filter that captures the soot that would otherwise be discharged into the air. The aspect which makes this system complicated is that soot understandably builds up in the filter and must be burned off with a ‘regen’ cycle. This requirement is complicated for vehicles that do not travel for long distances. The systems are heavily computerized to monitor and trigger ‘regen’ cycles. Again the combination of controlling gas under high heat and pressure with a computer system makes this process expensive and fragile.

The EPA does not mandate any particular technology, but rather emissions levels that vehicles must meet. Manufacturers then choose which technologies to use, but most manufacturers use all three of the above.

The question as to why these systems are so expensive to fix compared to any other vehicle system probably has a lot to due with because it’s required. If this was something you didn’t necessarily have to get fixed, it seems likely prices would be lower.