Which technician is correct about diesel oxidation catalysts and low-temperature operation?

• A. Technician A
• B. Technician B
• C. Both
• D. Neither

Answer: (B)

Diesel oxidation catalysts rely on catalytic reactions that need sufficient heat to reach their light-off temperature, so they operate best when the exhaust is hot. At low exhaust temperatures, the rate of the oxidation reactions is too slow, so the catalyst doesn’t convert CO and hydrocarbons (and some NO) efficiently. That’s why a technician who emphasizes reduced DOC effectiveness during cold starts or low-temperature operation is pointing to the correct limitation of DOCs. The light-off temperature for a DOC is typically reached only after the exhaust has heated up to a few hundred degrees Celsius, so during cold-starts the catalyst is not at its peak performance. Elements like sulfur in fuel or aging can further reduce activity and raise the effective light-off temperature, making low-temperature operation even less efficient. The other technician’s claim would be inconsistent with this fundamental behavior of DOCs.