ASE A1 Engine RepairFuel, Emissions, and Ignition Systems (General)Easy
A vehicle fails an emissions test due to high hydrocarbon (HC) and carbon monoxide (CO) levels. The technician performs a visual inspection and finds no obvious issues. Further diagnosis reveals that the engine is running rich. Which of the following is the most likely cause?
- AA vacuum leak.
- BA lean air-fuel mixture.
- CA restricted exhaust system.
- DA faulty engine coolant temperature (ECT) sensor reporting a cold engine.
Show answer & explanationAnswer & explanation
Correct answer: D. A faulty engine coolant temperature (ECT) sensor reporting a cold engine.
High HC and CO emissions, especially when the engine runs rich, indicate incomplete combustion. If the ECT sensor falsely reports a cold engine, the ECU will enrich the fuel mixture for warm-up, leading to a consistently rich condition and high HC/CO.
Why the other options are wrong
- A. A vacuum leak causes a lean condition, leading to positive fuel trims and potentially high NOx, not high HC and CO from a rich condition.
- B. A lean mixture would typically result in high NOx, not high HC and CO.
- C. A restricted exhaust system can affect engine performance but doesn't directly cause a rich condition and high HC/CO in this manner.
High HC/CO Emissions (Rich)
High levels of unburned hydrocarbons (HC) and carbon monoxide (CO) in exhaust indicate incomplete combustion, often due to an overly rich air-fuel mixture.
- HC = unburned fuel, CO = partially burned fuel.
- High levels often mean engine is running rich.
- Common causes: faulty sensors, leaking injectors, incorrect fuel pressure.
Memory trick: HC/CO are 'High' when fuel is 'R-I-C-H'.