Exhaust Emissions Testing — Idle CO Measurement Conditions Under which test condition should the idle carbon monoxide (CO) percentage be measured for a vehicle?

Introduction / Context:
Idle CO measurement is a basic emissions diagnostic for spark-ignition engines. To obtain repeatable and comparable results, standardized test conditions are used to minimize load variations on the engine during the measurement.

Given Data / Assumptions:

• Engine at normal operating temperature.
• Vehicle idling at specified rpm (often 600–900 rpm as per manufacturer).
• No additional vehicle loads unless specified by the test protocol.

Concept / Approach:
Turning on electrical consumers (headlights, blower, rear defogger) raises alternator load, which can alter idle rpm and mixture control, skewing CO readings. Therefore, the usual requirement is to perform the idle CO test with accessories off to reflect baseline engine-out emissions rather than accessory load response.

Step-by-Step Solution:
Stabilize engine at operating temperature.Ensure no auxiliary loads: lights and accessories off.Insert probe into tailpipe; measure CO at specified idle speed.Record value as idle CO percentage.

Verification / Alternative check:
Inspection procedures in many regions stipulate accessories off during idle emissions checks, sometimes with the radiator fan cycling allowed if automatic.

Why Other Options Are Wrong:
Switching on loads increases alternator drag and can change mixture control, compromising standardization.Headlights-only or blower-only options still represent added load and are not the default baseline condition.

Common Pitfalls:
Measuring immediately after cold start; a warmed-up engine is needed for consistent readings.

Final Answer:
Headlights and all other electrical devices switched off