Automotive Service Excellence (ASE) A8 Engine PerformanceFuel, Air Induction, and Exhaust Systems Diagnosis and RepairHard
A technician is performing an exhaust gas analysis on a vehicle. The readings show consistently high levels of hydrocarbons (HC) and carbon monoxide (CO), but low levels of nitrogen oxides (NOx). The oxygen sensor readings are normal. Which of the following is the MOST likely cause?
- AA restricted catalytic converter.
- BA lean air-fuel mixture.
- CAn exhaust leak before the oxygen sensor.
- DA rich air-fuel mixture.
Show answer & explanationAnswer & explanation
Correct answer: D. A rich air-fuel mixture.
High HC (unburnt fuel) and CO (partially burnt fuel) indicate a rich air-fuel mixture, meaning there's not enough oxygen for complete combustion. Low NOx is also consistent with a rich mixture because higher combustion temperatures (which are reduced in rich conditions) produce more NOx. Normal O2 sensor readings might mislead but don't negate the direct combustion product evidence.
Why the other options are wrong
- A. A restricted catalytic converter would likely cause high HC, CO, *and* NOx if the catalyst isn't converting them, and might also cause power loss. This scenario specifically mentions low NOx.
- B. A lean air-fuel mixture would typically result in low HC and CO, but high NOx due to higher combustion temperatures.
- C. An exhaust leak before the oxygen sensor would introduce ambient air, making the O2 sensor read lean, leading to the ECM enriching the mixture, which could cause high HC/CO, but the scenario states normal O2 readings, making this less likely as the primary cause.
Rich Mixture Exhaust Analysis
Exhaust gas analysis results indicating an air-fuel mixture with too much fuel relative to air, leading to incomplete combustion.
- High Hydrocarbons (HC) from unburnt fuel.
- High Carbon Monoxide (CO) from incomplete combustion.
- Low Nitrogen Oxides (NOx) due to reduced combustion temperatures.
Memory trick: HC/CO high, NOx low, rich mixture's flow.