Automotive Service Excellence (ASE) A8 Engine PerformanceEmissions Control Systems Diagnosis and RepairHard
A technician is diagnosing a vehicle that has failed an emissions test due to high Carbon Monoxide (CO) and Hydrocarbon (HC) readings at idle and 2500 RPM. The vehicle also has a 'Check Engine' light illuminated with DTC P0172, 'System Too Rich (Bank 1)'. After confirming adequate fuel pressure and injector spray patterns, what is the MOST likely next component to inspect?
- AExhaust Gas Recirculation (EGR) valve.
- BOxygen (O2) sensor.
- CMass Air Flow (MAF) sensor.
- DCanister purge valve.
Show answer & explanationAnswer & explanation
Correct answer: C. Mass Air Flow (MAF) sensor.
A rich condition (P0172) with high CO and HC often indicates too much fuel or not enough air. Since fuel pressure and injectors are good, the MAF sensor is a prime suspect. If the MAF sensor is reporting a higher-than-actual airflow, the ECU will command too much fuel, leading to a rich condition and high CO/HC.
Why the other options are wrong
- A. An EGR valve issue typically affects NOx emissions or causes misfires/rough idle, not primarily a rich condition with high CO/HC.
- B. While an O2 sensor can report a rich condition, a faulty O2 sensor itself causing a rich condition would typically lead to a P0172 only if it erroneously reported a lean condition, causing the ECU to add fuel. If the O2 sensor is accurately reporting a rich condition, it's a symptom, not the cause.
- D. A leaking canister purge valve could cause a rich condition, but it's less likely to cause consistently high CO/HC at both idle and 2500 RPM compared to a MAF sensor, and may have a different DTC.
MAF Sensor Over-Reporting
A Mass Air Flow (MAF) sensor that provides an erroneously high airflow reading to the Engine Control Unit (ECU), causing the ECU to inject excessive fuel and create a rich fuel mixture.
- Leads to DTC P0172 (System Too Rich).
- Results in high CO and HC emissions.
- Causes include contamination or electrical malfunction.
Memory trick: MAF's Mistake: More Air, More Fuel, More Trouble.