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Sunday, December 1, 2013

How Can a Bad Gas Cap Cause a Check Engine Light?

Why Is It Important to Tighten Your Gas Cap?

When you replace your gas cap after filling up at the gas station, you have probably noticed that the gas cap clicks as you tighten it.  There is usually a sticker or a note on the gas cap that indicates to tighten it at least 3 clicks.  Some vehicles have a note that says the check engine light may come on if you do not tighten the gas cap at least 3 clicks.  Why is the engine so sensitive to the gas cap?

A Bad Gas Cap Can Cause the Check Engine Light to Turn On
Image Source: Dr. Penny Pincher

The Gas Cap Helps Capture Fuel Vapors

The gas cap is part of the evaporative emission control system on a vehicle.  In the old days, gas caps simply had a small hole to allow fuel vapor to vent.  Modern vehicles have a system in place to capture these fuel vapors and prevent uncombusted hydrocarbons from escaping into the atmosphere.  Fuel vapors from the gas tank are routed through vacuum lines and the fuel vapor is condensed and fed into the engine, allowing the fuel to be burned in the vehicle's engine.

Why Does a Bad Gas Cap Cause the Check Engine Light to Turn On?

The evaporative emission control system has sensors to monitor for leaks to detect if fuel vapor is escaping.  If pressure is not maintained in the system, the check engine light comes on to indicate that fuel vapor is leaking.  One source of a leak can be a bad gas cap, a missing gas cap, or a gas cap that is not tightened fully.

Check Engine Light from Evaporative Emision Control System Fault- Bad Gas Cap
Image Source: Dr. Penny Pincher


Can Replacing the Gas Cap Turn Off the Check Engine Light?

Since tightening or even replacing a gas cap is easy and inexpensive, it is often the first thing to try to resolve a check engine light related to the evaporative emission control system.  You can use a OBD II code reader or scan tool to find information about the cause of the check engine light fault in your vehicle.  There are several codes related to the evaporative emission control system that could be caused by a bad gas cap:
  • P0440
  • P0455
  • P0442
A Bad Gas Cap can Cause the Check Engine Light to Stay On
Image Source: Dr. Penny Pincher


Diagnostic Scan Tool from Amazon- read your own engine fault codes on any car 1996 or newer...




If you have one of these faults, make sure you have tightened your gas cap at least 3 clicks.  If your cap is tight and you still have the check engine light on, consider replacing the gas cap.  The cost of a replacement gas cap is about $15.  There are many possible causes for evaporative emission faults, but you can easily replace the gas cap yourself for a few dollars.  If you are lucky, this will resolve your check engine light.

The on-board computer may require a number of starts without detecting a fault to clear the check engine light, so it may take a few weeks to see if a new gas cap cleared the fault.  You can use a OBD II diagnostic scan tool tool to clear the fault.  This will turn off your check engine light so you can easily see if it returns or not with a new gas cap.



If tightening your gas cap or replacing your gas cap with a new one does not clear your check engine light, you may need to take your vehicle to a mechanic to diagnose the problem.

How I Made My Check Engine Light Turn Off By Replacing the Gas Cap

The check engine light on my car came on.  I used a code scanner to check the fault code and found the following fault codes:

  • P0440 Evaporative Emission
  • P0441 Canister Purge Valve

These fault codes can be caused by a number of issues with the evaporative emission control system, including the pressure pump, vacuum line leaks or a bad solenoid.  A faulty gas cap can also cause these faults.  I noticed that the gas cap was not clicking very firmly when I tightened it.  I decided to try replacing the gas cap since it was easy to do and does not cost very much.

I spent a few dollars extra to get an OEM gas cap from a dealer parts counter.  The advantage was that this gas cap included the plastic line that hooks onto the inside of the fuel door so you can allow the gas cap to hang while you are filling up.  The disadvantage of buying an OEM fuel cap was that it cost $22.  A generic gas cap costs about $5.

The result:  My check engine light turned off within a few days after replacing the gas cap.


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