How does a saturated charcoal canister affect 4-gas readings during a TSI Smog Check?

• A. It would result in low HC and CO at idle.
• B. It would have no effect on 4-gas readings.
• C. It would cause unstable readings in all gases.
• D. It would result in high HC and CO at idle.

Answer: (D)

A saturated charcoal canister leads to more fuel vapors entering the engine instead of being held back. Those extra vapors burn along with the air-fuel mixture, which increases the amount of unburned hydrocarbons in the exhaust, especially at idle when purge flow and engine vacuum are changing rapidly. The increased hydrocarbon output can also push the mixture toward a slightly richer condition, which in turn raises carbon monoxide in the exhaust. So the most likely result during a TSI Smog Check is higher HC and CO readings at idle. This wouldn’t produce no effect or instability in all gases; the direct, measurable impact is an rise in HC and CO due to the extra vapors being burned.