Which signs indicate a problem with the exhaust aftertreatment system on a diesel bus?

• A. Decreased brake wear indicator.
• B. Increased engine oil pressure.
• C. Improved fuel economy and power.
• D. Check engine light, reduced power, poor fuel economy, frequent regen cycles, excessive soot.

Answer: (D)

Problems in the exhaust aftertreatment system show up as warning lights and running symptoms because the engine control system monitors temperatures, pressures, and sensor signals to protect the aftertreatment components. When something isn’t right, you’ll typically see a check engine light accompanied by power derating, since the ECU limits engine output to prevent damage. Fuel economy tends to drop because the system adds extra regen events or the engine runs in a less efficient mode to manage soot and catalysts.

Frequent regen cycles happen when the diesel particulate filter is nearing saturation; the system tries to burn off accumulated soot more often, which increases fuel use and can cause noticeable performance changes. Excessive soot buildup itself is a clear sign the aftertreatment isn’t clearing particulates properly, indicating a problem with the system, its sensors, or the combustion process feeding it.

The other options don’t reflect aftertreatment faults. A sign like decreased brake wear is unrelated to exhaust aftertreatment. An increase in engine oil pressure isn’t a typical indicator of aftertreatment trouble, and improved fuel economy with more power would suggest normal or better-than-normal operation, not a fault.