ASE A1 Engine RepairGeneral Engine DiagnosisMedium
A customer complains of a sudden, significant drop in fuel economy and a strong smell of raw fuel from the exhaust. The engine runs rough and the check engine light is on. A scan tool reveals a P0420 code (Catalyst System Efficiency Below Threshold Bank 1). Which of the following is the MOST likely root cause?
- AFaulty mass air flow (MAF) sensor
- BLeaking fuel injector
- CClogged fuel filter
- DWorn spark plugs
Show answer & explanationAnswer & explanation
Correct answer: B. Leaking fuel injector
A leaking fuel injector would cause an excessive amount of raw fuel to enter the combustion chamber, leading to a rich condition. This results in poor fuel economy, rough running, a raw fuel smell from the exhaust, and can quickly damage the catalytic converter, triggering a P0420 code.
Why the other options are wrong
- A. A faulty MAF sensor can cause various drivability issues and fuel economy changes, but a rich condition with a strong raw fuel smell and P0420 is more indicative of direct fuel over-delivery.
- C. A clogged fuel filter would typically lead to a lean condition, engine misfires, and potentially stalling, but not a strong raw fuel smell or rich condition.
- D. Worn spark plugs would cause misfires and reduced power, but generally do not result in a strong raw fuel smell from the exhaust unless combined with other issues causing severe unburnt fuel.
Rich Fuel Condition
An engine operating state where there is an excessive amount of fuel compared to air in the air-fuel mixture.
- Causes include leaking injectors, stuck-open fuel pressure regulators, or faulty O2 sensors.
- Symptoms: poor fuel economy, black exhaust smoke, raw fuel smell, rough idle, fouled spark plugs.
- Can damage catalytic converters due to unburnt fuel.
Memory trick: Rich fuel's bad smell, like a leaky fuel cell.