Automotive Service Excellence (ASE) A8 Engine PerformanceEmissions Control Systems Diagnosis and RepairMedium
A technician is performing an emissions test on a vehicle. The test results show high levels of Hydrocarbons (HC) and Carbon Monoxide (CO), along with low Oxygen (O2) in the exhaust stream. The 'Check Engine' light is off, and there are no DTCs. What is the MOST likely cause of these readings?
- AA properly functioning Air Injection Reaction (AIR) system.
- BA rich air-fuel mixture.
- CA lean air-fuel mixture.
- DAn efficient catalytic converter.
Show answer & explanationAnswer & explanation
Correct answer: B. A rich air-fuel mixture.
High HC and CO, combined with low O2, are classic indicators of a rich air-fuel mixture. The engine is receiving too much fuel or not enough air, leading to incomplete combustion.
Why the other options are wrong
- A. A properly functioning AIR system would inject air to help oxidize HC and CO, which would typically lower these emissions and increase O2, not result in high HC/CO and low O2.
- C. A lean mixture would typically result in high NOx and O2, with lower HC and CO (unless misfiring).
- D. An efficient catalytic converter would reduce HC, CO, and NOx, leading to lower readings for those emissions.
Rich Mixture Emissions
A rich air-fuel mixture results in high levels of Hydrocarbons (HC) and Carbon Monoxide (CO) in the exhaust due to incomplete combustion, with low residual Oxygen (O2).
- Too much fuel, not enough air.
- HC = unburnt fuel, CO = partially burnt fuel.
- Low O2 because available oxygen is consumed in combustion.
Memory trick: Rich means fuel overload: HC/CO soar, O2 disappears!