ASE A1 Engine RepairFuel, Emissions, and Ignition Systems (General)Hard
A vehicle fails an emissions test due to high HC (hydrocarbons) and CO (carbon monoxide) levels. The technician checks for DTCs and finds none. Which of the following is the most likely cause?
- AA clogged exhaust gas recirculation (EGR) valve
- BAn excessively rich air-fuel mixture or engine misfires
- CA lean air-fuel mixture due to a vacuum leak
- DA faulty catalytic converter
Show answer & explanationAnswer & explanation
Correct answer: B. An excessively rich air-fuel mixture or engine misfires
High HC (unburned fuel) and high CO (partially burned fuel) indicate an incomplete combustion process. This is typically caused by an excessively rich air-fuel mixture (too much fuel, not enough air) or by engine misfires where fuel is not burned at all. The absence of DTCs suggests the problem is not severe enough to trigger a fault code, or is a borderline condition.
Why the other options are wrong
- A. A clogged EGR valve primarily increases combustion temperatures, leading to higher NOx emissions, not directly high HC and CO.
- C. A lean mixture (too much air) would typically cause high NOx and potentially high O2, not high HC and CO.
- D. While a faulty catalytic converter contributes to high HC/CO, the initial cause is usually incomplete combustion, which the catalyst then fails to process. The question asks for the 'most likely cause' of the *levels*, implying the condition *before* the catalyst fails to clean them.
High HC/CO Emissions
Elevated levels of unburned hydrocarbons and carbon monoxide in exhaust gases, indicating incomplete combustion of fuel.
- Caused by rich mixtures, misfires, or ignition problems.
- Often points to issues with fuel delivery, ignition, or engine mechanicals.
- Catalyst can be overwhelmed by these conditions.
Memory trick: HC/CO high, incomplete burn, NOx high, too hot to learn.