Automotive Service Excellence (ASE) A8 Engine PerformanceFuel, Air Induction, and Exhaust Systems Diagnosis and RepairMedium
A customer complains of poor fuel economy and black smoke from the exhaust. During diagnosis, a technician observes that the upstream oxygen sensor (O2S1) voltage fluctuates slowly between 0.1V and 0.9V, but the downstream oxygen sensor (O2S2) voltage is consistently high at 0.8V. Which of the following is the MOST likely cause?
- AA lean running condition.
- BA leaking exhaust manifold.
- CA clogged catalytic converter.
- DA rich running condition.
Show answer & explanationAnswer & explanation
Correct answer: D. A rich running condition.
Black smoke from the exhaust is indicative of a rich fuel mixture. An upstream O2 sensor fluctuating correctly indicates the engine is attempting to regulate the mixture. A consistently high downstream O2 sensor voltage (0.8V) indicates that the catalytic converter is being overwhelmed with unburnt fuel (a rich condition), as it's detecting low oxygen content after the converter.
Why the other options are wrong
- A. A lean condition would typically result in white smoke (if any), and downstream O2 sensor voltage would be low (indicating high oxygen post-catalyst).
- B. A leaking exhaust manifold would introduce outside air, causing the O2 sensors to read lean, which is contrary to the observed black smoke and high downstream O2 voltage.
- C. A clogged catalytic converter would primarily cause reduced engine power, overheating, and potentially a rotten egg smell, but not typically a consistently high downstream O2 voltage with black smoke.
O2 Sensor Readings (Rich)
A rich condition is indicated by black smoke and a downstream oxygen sensor consistently reading high voltage (above 0.7V), as the catalytic converter is overwhelmed and cannot store oxygen.
- Black smoke is a visual indicator of richness.
- Upstream O2 should still switch if engine tries to compensate.
- Downstream O2 stays high due to excess unburnt fuel.
Memory trick: High downstream voltage + black smoke = too much gas.