When letting off the gas and stopping, why does check engine light come on?

The gas cap misalignment is the most often cause of the check engine light illumination. The fuel system is pressurized, and there is a special filter used to capture fumes and condense them into liquid and put them back in your gas tank. So a loose gas cap stops this from happening and causes gas vapor to be let out into the atmosphere. If the light doesn't come on again, you're probably good to go. If it does, and from you r description, it appears to be connected with the gas cap, you can pretty cheaply buy another one from Autozone. Also, there are parts places, like I think O'Reilly's who will plug in a computer for free to your car to tell exactly the cause of the check engine light. Could be many things.....

you check the gas filter when you have a bad one its run to a point the gas cant go throgh its stalls pump stop gas go back and start over again pressure push the dirt again let me no pierre

you did not give a trouble code from the check engine light, but probably a Vacuum leak.

check fuel pressure, bad pressure relay or pump

run the fault codes again and fix the problem sensor

When you get a check engine light it means a fault has occurred and a fault code for that is in memory, now you need the fault codes or codes read to see what is going on and then troubleshoot the system.

The check engine light flashing mean the engine is misfiring. Sound like you need a tune-up.

There are so many things that cause a check engine light ot come on that its almost impossible to tell you how to fix it. If you can stop by an Autozone or Advance auto parts store they will usually put a code tester on your car and get the error codes stored in your cars computer. In a lot of cases there are vacuum leaks. Vacuum lines become brittle and crack letting excell air into your intake system. If your gas cap gasket is worn, same thing. You also have a catalytic converter that may be clogged up as well as orygen sensors in the exhaust system that may have failed. There is an exhaust gas recirculator that opens when the computer calls for exhaust gas, as well as a positive crankcase ventilator that may be stopped up. Check engine lights are just an indicator that one or more of these things have quit working.

this is a very common problem. but mostly the gas cap just wasn't tightened properly. The check engine light will reset itself if the computer detects the problem has been fixed. This may, however, take many start stop cycles. Many auto parts stores will check your computer for codes at no cost. This will let you know if you are fixing the correct problem

check ur air filter sensor and other sensors as well. had similar problems with mine