Dashboard warning lights are sometimes useful, sometimes important, and always annoying. The "check fuel cap" indicator is no exception. It's a common warning light on the GMC Sierra that can be caused by a number of different issues, but it's usually not a big deal. Here's how to turn it off without much hassle.
What is a Fuel Cap?
First things first, what is a fuel cap? Many people take this knowledge for granted, but if you're not familiar with cars or don't speak english as your first language, it can be a source of confusion. The fuel cap is the part of the fuel filler assembly that you unscrew to fill up your car with gas. It's usually made of plastic, but can also be made of metal. It typically resembles a screw – and recently some higher-end vehicles have eschewed them entirely in favor of a capless system.
The fuel cap is an important part of your vehicle, ensuring that fuel doesn't spill out of the tank or evaporate into the air (this is more important than you might think!) The fuel cap also prevents contamination from dirt, dust, and other debris from entering the fuel tank. It's easy & also important to keep your fuel cap clean & in good condition.
What Does The Check Fuel Cap Light Mean On A GMC Sierra?
The "check fuel cap" light is a warning that the fuel cap is not properly installed. It's a common warning light on the GMC Sierra and is usually caused by a loose or missing fuel cap. It can also be caused by a faulty fuel cap, a faulty fuel filler neck, or a faulty fuel tank pressure sensor. Generally, however – the issue is that you just haven't screwed it in properly or at all.
If you've checked & it's installed & seated correctly, take a rag & wipe down both the inside part of the fuel cap & the fuel cap neck that receives it. This will remove any dirt or debris that may be preventing the cap from seating properly. If that doesn't work, you may need to replace the fuel cap. We recommend buying parts from Amazon as you'll be able to get the exact one you need, instead of a generic cap from, e.g., AutoZone that typically won't fit quite right & may not fix the issue of the light being on.
Fuel caps for the GMC Sierra can be purchased here if you determine a replacement is required.
How Does A GMC Sierra Know That Its Fuel Cap Is Missing?
Your GMC Sierra has a fuel cap pressure sensor that monitors the pressure inside the fuel tank. When the fuel cap is installed, the sensor detects the pressure & the Check Fuel Cap light goes off. When the fuel cap is missing, the sensor detects a drop in pressure & the Check Fuel Cap light comes on. This is a safety feature that addresses both fuel evaporation & spillage concerns.
How To Fix The Check Fuel Cap Light On A GMC Sierra
The first task when diagnosing & fixing a Check Fuel Cap light is to visually & manually inspect the fuel cap. Look at it to ensure it's seated, then try screwing it in by hand to make certain it's not just loose. If the cap feels "sticky" or "stuck" when you try to screw it in, it may be that the cap is dirty & needs to be cleaned. If the cap is clean & still won't screw in, it may be that the cap is damaged & needs to be replaced.
One area to pay special attention to is your gas cap's "O Ring" which is a circular rubber seal that helps your gas cap seal properly. If the O Ring is damaged or missing, it can cause the Check Fuel Cap light to come on. If you suspect the O Ring is damaged, we recommend replacing the entire gas cap assembly instead of trying to repair or replace the O Ring alone.
If All Else Fails…
There's a small chance your GMC Sierra's gas cap is actually fine & the indicator light or the electrical control unit is the problem. I want to stress, this is exceedingly rare but it can happen. If you've tried everything else & the light is still on, you may need to replace the fuel cap indicator light or the electrical control unit that controls it. This is usually a job for a professional mechanic, so we recommend taking your vehicle to either a dealership or a shop familiar with electrical work if you're not already an expert.
Conclusion & Our Standard Caveat:
The "Check Fuel Cap" indicator on GMC Sierra is an annoying reminder that is typically an easy thing to fix. There's not many ways to do this wrong, but we remind you – Use the information above at your own risk. This article is not meant to be a substitute for professional advice. If you're not comfortable with the steps outlined above, we recommend taking your GMC Sierra to a mechanic or dealership to have them diagnose & repair the issue for you.