Evaporative Emission Control System (EVAP) – Function – Failure

Evaporative Emission Control System (EVAP) - Function - Failure
Evaporative Emission Control System (EVAP) - Function - Failure

The function of the evaporative emission control system (EVAP), is to trap and store evaporative fuel emissions.

The (EVAP) system therefore, totally eliminates fuel vapors, as a source of air pollution.

But, sealing the fuel tank is not as simple as it sounds. For one thing, a fuel tank must have some type of venting, so air can enter to replace the consumed fuel.

Firstly, the evaporative emission control (EVAP) system, captures gasoline fumes and other emissions. So, if the fuel tank was sealed tight, the fuel pump could create enough negative pressure, to collapse it. Consequently, on older (EVAP) systems, the tank is vented, by a spring-loaded valve, inside the gas cap. While, on newer vehicles it is vented, through the (EVAP) canister.

Evaporative Emission Purge Control Valve And Solenoid
Evaporative Emission Purge Control Valve And Solenoid

Consequently, A typical system consists of:

  • A small canister full of charcoal
  • Valves
  • Hoses
  • Vents in the fuel lines
  • A sealed fuel tank cap

When fuel evaporates inside the gas tank, the excess vapors, are transferred to the charcoal canister. They’re stored there, until they can be, safely transferred back to the engine. Then, burned with the normal air-fuel mixture.

The most obvious symptom of an evaporative emission control (EVAP) system problem, is the Check Engine light (CEL). A common cause of this, is a loose gas cap. So, if you’ve recently filled up on fuel, check that the gas cap is fully tightened.

Consequently, if the Check Engine light (CEL) remains on after checking the gas cap, there may be a bigger problem.

Evaporative Emission Control System (EVAP) Components:

  • Fuel Tank
  • Fuel Cap
  • Liquid-Vapor Separator
  • (EVAP) Canister

Common Problems – Evaporative Emission

All (EVAP) systems have a charcoal canister that it relies on. The canister has a huge surface area, that is effective in storing vapors from the fuel.

So, if the charcoal canister where to go bad, there would be an array of problems. In addition, to power reduction, fuel odor, more pinging, lower fuel economy, and flooding.

So, there are some common problems, with the (EVAP) system. Most often, with the purge valve, that vents fuel vapors to the engine. The canister purge valve, also commonly referred to as the purge valve, is an important component of your (EVAP) system. Consequently, this system restricts the fuel vapors created in your fuel tank, from escaping into the atmosphere by trapping them. So, when your engine runs at regular speed. The evaporative emission control (EVAP) system, slowly allows these vapors back into your engine.

Purge Valve
Purge Valve

So, the canister purge valve, controls the flow of these vapors. Consequently, the valve regulates, when and how much of these vapors enter your engine. Another name for the canister purge valve, is the solenoid.

The most common purge valve issues are:

  • Stuck open.
  • Stuck closed.
  • It may just not open, at the proper time.

Canister Purge Valve, Evaporative Emission Control (EVAP) System Symptoms:

Check Engine Light Is On

  • So, the first sign of trouble for your canister purge valve is, the check engine light coming on. If your car’s computer detects higher or lower than expected purging, it will illuminate the check engine light. As a result, setting off error codes.

Purge Valve Stuck Closed

  • Also, if your canister purge valve is not opening properly, it may adversely impact your gas mileage. Consequently, the vapors that your car regularly uses, will be vented to the environment. As a result, you will lose a portion of the fuel, regularly used in burning.

Purge Valve Stuck Open

  • So, if your canister purge valve is stuck open, a vacuum leak is created. And, that leak can affect your engine adversely. Simply put, air will be allowed to enter the engine, in a quantity that is not predicted by your computer. This will change the car’s air to fuel ratio, and can cause rough idling, as well as difficulty starting.
  • Therefore, if experienced together, there is a high likelihood, of a failing canister valve. Furthermore, this enriches the fuel mixture and will likely, foul the spark plugs. So, most charcoal canisters, have a replaceable filter.
Evaporative Emission Charcoal Canister
Evaporative Emission Charcoal Canister

Possible Fault Codes – Evaporative Emission Control System (EVAP):

  • P0440….Evaporative Emission Control System Fault
  • P0441….Evaporative Emission Control System Incorrect Purge Flow
  • P0442….EVAP Emission Control System Leak Detected (small leak)
  • P0443….EVAP Emission Control System Purge Control Valve Circuit
  • P0444….EVAP Purge Control Valve Circuit Open
  • P0445….EVAP Purge Control Valve Circuit Shorted
  • P0446….Evaporative Emission Control System Vent Control Circuit
  • P0447….EVAP Emission Control System Vent Control Circuit Open
  • P0448….EVAP Emission Control System Vent Control Circuit Shorted
  • P0449….EVAP Emission Control System Vent Valve/Solenoid Circuit
  • P0450….Evaporative Emission Control System Pressure Sensor
  • P0451….EVAP Emission Control System Pressure Sensor
  • P0452….EVAP Emission Control System Pressure Sensor Low Input
  • P0453….EVAP Emission Control System Pressure Sensor High input
  • P0454….EVAP Emission Control System Pressure Sensor Intermittent
  • P0455….EVAP Emission Control System Leak Detected (gross leak)
  • P0456….EVAP Emission Control System Leak Detected (small leak)
  • P0457….EVAP Emission Control System Leak Detected (fuel cap)


Firstly, the (EVAP) emission control system, prevents the escape of fuel vapors, from a vehicle’s fuel system. So, when the engine is running, a purge valve opens, allowing vacuum to siphon the fuel vapors into the engine.

In addition, fixing (EVAP) codes, can be a challenge, even for professional technicians. Finally, if you have, a P0442 small leak code, you may need, a smoke machine to find it.

Thank You !