A dilution error in the Emission Inspection System indicates the system is reading high oxygen. Which option matches this statement?

• A. The dilution error indicates high O2
• B. The dilution error indicates low O2
• C. The dilution error indicates high CO
• D. The dilution error indicates high NOx

Answer: (A)

The key idea is how dilution sampling affects what the analyzer sees. In a dilution system, exhaust gas is mixed with ambient air to bring concentrations into the sensor’s usable range. If the dilution process is off—too much ambient air is being mixed in or the flow isn’t controlled correctly—the sample contains more oxygen from the surrounding air than the actual exhaust. That makes the O2 sensor read a higher oxygen level than what’s truly present in the exhaust. So a dilution error is interpreted as high O2. In practice, this kind of fault can also make CO and HC readings appear lower because those pollutants are further diluted, while NOx levels are less directly affected by the dilution in the same way.