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235 1/2 Carron Street

Pittsburgh, PA 15206

Phone: 412-361-3554


Open Mon-Fri - 7:30 to 5:00pm


What is an evaporative (EVAP) system?


Gasoline vapors are considered a pollutant. Clean air standards dictate that every vehicle needs a system in place to control the release of hydrocarbons (fuel vapor) into the atmosphere.

If you are old enough to remember when cars didn’t have extensive evaporative control systems, whenever you would fill your gas tank you would smell a very strong odor of gasoline as the gas vapors in the tank were displaced with the liquid fuel. With modern cars there is very little gas odor because the fuel vapor is being stored in a charcoal canister to be burned by the engine at a determined time.

The basic concept of the system is to store fuel vapor and then burn them in the engine so as not to cause excessive emissions either from the raw fuel vapor or from the exhaust. For federal emission regulation reasons the system is tested by your vehicle’s engine computer (PCM) to ensure it is operating within spec. If the EVAP system is not operating correctly the Check Engine (Service Engine Soon) light will illuminate.

The most common problems with these EVAP systems are leaks. The diagnostic system will use positive or negative (Vacuum) pressure to determine is the EVAP system is leaking. The EVAP System includes the gas tank, control valves, sensors and all related piping. The most common cause of a leak is the gas cap was not properly reinstalled the last time the vehicle was fueled, causing the system to leak. Due to the way the system is self-tested, the Check Engine light may not come on right away, but may take a day or two to turn on.

If the gas cap was left loose you can tighten the cap. The Check Engine light will remain on until the PCM checks the EVAP system and sees the system pass twice in a row, after which, the Check Engine light will extinguish. Basically, if the Check Engine light is on and the engine is running normally is it safe to drive the vehicle. If the Check Engine light is flashing do not continue driving the vehicle. A problem in the EVAP system will cause no ill effects on engine performance.

We hope this answers your questions on automotive evaporative systems.

Bastone Auto Service LLC

Question: My Check Engine (Service Engine Soon) light was on. My mechanic repaired my vehicle but now I need to “Set the monitors” so that they can perform the PA state emissions test. What are these monitors and why do I need to drive my car to “Set” them?


Answer: This is a very common situation. The way the laws are written for PA and federal emissions testing the Service Engine Soon light cannot be illuminated during the emissions test procedure. The Service Engine Soon light indicates a malfunction in your vehicle’s emission system and must be repaired before the vehicle can pass the PA state emissions test.


Modern vehicles are very sophisticated and there are many activities going on within you vehicle’s electrical system that the driver is completely unaware. While you are driving down the road your engine’s brain, the Powertrain Control Module (PCM) is performing actual tests on your engine to see if all sensors and actuators are performing to specification. These self-tests are completely transparent and you don’t know when the actual test on a given system is occurring. While every vehicle is different, these system tests include items such as:


 -Catalytic converter efficiency

 -Engine misfire

 -Evaporative fuel system

 -Exhaust Gas Recirculation (EGR) system

 -Air Injection Reactor system (AIR)

 -Oxygen sensors and oxygen sensor heaters


Each of these systems are controlled by “Monitors”. The monitor is listed as either “Ready” or “Not ready for test”. After vehicle repairs, the technician clears any stored trouble codes in the PCM. When the codes are cleared all monitors are set to “Not ready for test”. The vehicle needs driven to allow the PCM to run the self-test in each system. After the test on a given system is run and passes, the monitor will now be set to “Ready”.  There are specific conditions the PCM will be looking for before it tries to run each monitor: For example, typically, the PCM will want to see the fuel level in your gas tank between 20% and 80% to run the EVAP system monitor. Typically PCM will want to see you driving approximately 40 MPH or more at a steady state for a given amount of time to run the catalytic converter monitor. You may need to decelerate without touching the accelerator pedal for a given amount of time to run the EGR monitor. Every vehicle and every manufacturer has different requirements to allow each monitor to run and set to “Ready”. It is said that you need to run the “Drive Cycle” to set all the monitors. However, unless you happen to have access to a test track it is very difficult to obtain the requirements a manufacturer lists as the drive cycle on public roads. To make matters more difficult, sometimes the specified drive cycle may not work due to firmware upgrades in your vehicle’s PCM. Some cars set their monitors very quickly while some cars it can be VERY difficult to get the monitors set to Ready. Again, each vehicle is different.



If your vehicle is model year 2000 or less, you can pass PA state emissions test with two monitors un-set.

If your vehicle is model year 2001 or higher you can have one monitor not set.


These are all PA state and/or federal requirements and rules. While they may be inconvenient for you the customer, we need to deal with this on a daily basis. It can be a challenge and your patience is appreciated.


We hope this answers your questions about emission monitors

Bastone Auto Service LLC