Signs Your Charcoal Canister Is Failing

The charcoal canister is the centerpiece of the evaporative emission control (EVAP) system, a technology designed to prevent vapors from your fuel tank from escaping into the atmosphere.

The charcoal canister seldom fails and should last the life of the car. In most cases, EVAP system problems are not caused by the canister itself but by related valves.

In some rare instances, however, the canister can crack or become clogged. Also, a failure somewhere else in the fuel or EVAP system can allow gasoline to enter the canister, ruining it.

Signs of a failing charcoal canister

These are the most common signs of a bad charcoal canister:

• Check engine light is on: If the car’s computer finds an EVAP system leak, including one from a cracked charcoal canister, it will turn on the check engine light. Likewise, it will turn on the light if it determines there isn’t enough airflow due to a clogged canister.
• Increased emissions: A damaged or clogged charcoal canister will prevent fuel vapors from being purged. Instead, the gases will be released into the atmosphere as harmful emissions.
• Fuel smell: A faulty charcoal canister, or one that’s been contaminated by gasoline, can result in a fuel smell inside or around the car.
• You can’t pump gas in your car: In some cases, if the charcoal canister is clogged or can’t vent, it can prevent the car from taking gas during refueling.
• Failed emissions inspection: Most states require annual emissions testing. A failed charcoal canister will trigger the check engine light and prevent a vehicle from passing this inspection.

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How the charcoal canister works

When the engine is off, harmful fumes are stored inside the canister by activated charcoal, which is more porous than regular charcoal. Once the vehicle is running and the engine is at regular speed, the vapors are “purged.” A valve opens to draw fresh air through the canister, carrying the gases into the engine, where they’re burned.

Today’s vehicles also feature a vent valve in the charcoal canister’s fresh-air hose. When the system needs to be tested for leaks, the valve is used to keep the canister shut. During purge, the valve opens to allow airflow.

These operations — purging, venting and system monitoring — are controlled by the car’s computer and are based on information it receives from sensors throughout the vehicle.

» LEARN MORE: Get an estimate for your charcoal canister replacement

How to fix the problem

A failed charcoal canister must be replaced to keep the check engine light off and limit pollution. But first, a thorough diagnosis of the EVAP system should be performed to ensure the canister is actually the problem.

A professional technician can do this using equipment such as a scan tool and smoke machine. The latter puts smoke through the EVAP system to pinpoint leaks.