What does the O2 sensor see when a non-PCM controlled ported vacuum controlled EGR valve is activated?

• A. The O2 sensor sees less oxygen in the exhaust.
• B. The O2 sensor sees more oxygen in the exhaust.
• C. The O2 sensor reads zero oxygen.
• D. The O2 sensor is unaffected.

Answer: (A)

When the EGR valve is activated, exhaust gas is redirected back into the intake, diluting the incoming air with inert exhaust components. That means less oxygen is available for combustion, so the exhaust that leaves the engine has a lower oxygen content. The O2 sensor sits in that exhaust and will respond to this reduced oxygen by showing a lower oxygen level (lower sensor voltage on a typical narrowband sensor). So the sensor sees less oxygen in the exhaust. The other possibilities—more oxygen, zero oxygen, or no change—don’t fit because EGR adds inert gas rather than oxygen and it does alter the exhaust composition.