Is gas cap causing the 'service engine soon' light to turn on?

Hello everybody, I’ve been having my ‘service engine soon’ light turn on in my 1999 Chevy Blazer. It has an automatic transmission and 60,000 miles. I first disconnected the battery to turn off the light and to see if it would turn on again. It turned on again so I took it to AutoZone for an obd2 scan and got codes P0300 and P0442. I replaced the spark plugs and spark plug cables and the light turned off, but then turned back on after a week. I’m starting to think the problem is something simple like the gas cap.

The rubber film around the cap has a few tears, but are the tears significant enough to cause the light to turn on?

Follow this link for a photo of my gas cap:

The P0300 code indicates “Random/Multiple Cylinder Misfire”, and P0442 indicates a small leak in the Evaporative Emissions System.

You really really need to have the scan done again, as replacing the plugs and cables may have resolved the misfire problem. It is very possible that the only remaining code will be the one for an evap system leak. If that is the case, replacing the gas cap could very possibly resolve the problem.

I don’t normally suggest just “throwing parts” at a problem but since a gas cap is relatively cheap and you do see some damage to the cap’s gasket, I would suggest replacing the cap. Then, drive the car for a couple of days to see if the CEL turns off. If you are very lucky, simply replacing the cap will do the trick. If not, you need to have a qualified mechanic do some diagnostic work.

Those cracks are enough, get a new cap and see if your light goes off.

You have to have the codes read again to find out if the P0300 is still there.

Buy a new gas cap. For Toyota it is important to get a true “Toyota” replacement from a dealer. Not sure about GM and how fussy they are about replacement gas caps. Generic “cheapie” gas caps aren’t going to work.

I don’t think a bad gas cap would throw a “service engine soon” light. It would cause a “check engine light” so if your truck has both CEL and SES lights separately I’d replace the cap but not be too surprised if the problem recurs.

It’s extremely likely that the gas cap is causing the CEL light. Your EVAP system monitors the vacuum in the tank, and if a leak allows it to disappear it trips a CEL.

I’ve attached an exploded view drawing of the system that might help to understand it. In the drawing, the
Fuel Tank Main Tube Sub Assembly goes to feed the injectors.
The Fuel Tank Vent Hose Sub Assembly goes to the Chacoal Canister,
the Fuel Tank Breather Tube goes to a small hole around the periphery of the gas fill tube opening. It allows fumes expelled while the tank is being filled to be drawn back into the bellows around the gas pump. It is NOT open to the outside world when the cap is installed, but is rather open to the fill pipe.

Note that a charcoal canister can be saturated in some older vehicles by “topping off” the tank. If this is your practice, it would be well to discontnue it. Whether it is or not, someone needs to look at the EVAP system.

Buy a new gas cap. For Toyota it is important to get a true “Toyota” replacement from a dealer.

This is true…Happened to me. I spent $15 on a aftermarket cap…light still kept coming on. Went the dealer and bought their cap for $35…no more light.

Similar problem w/Honda 05 CR-V code P0420 indicating an exhaust problem.,next time I fill gas tank I “clicked” the gas cap around a couple of turns and the light on the dash is now off!!

P0420 is the code cor “catalyst ystem Efficiency below threshold, bank 1”. Assuring good installation of the gas cap didn’t fix that. It was a simple coincidence.

That code, should it recur (and it probbaly will) truely is likely to be a catalytic converter functioning marginally.

‘service engine soon’ light turn on in my 1999 Chevy Blazer … I took it to AutoZone for an obd2 scan and got codes P0300 and P0442.

There are Service Engine lights and Check Engine Lights (CEL)

Service Engine Lights mean maintenance is due. It does not indicate there is a problem (unless you ignore it long enough and then there will be a problem.) The P0000 codes are Check Engine Lights.

Joseph, a code is a code is a code. What you may have experienced that illumination of the Service Engine Soon light caused you to have the ECU checked and you retreived some stored codes, even though the SES light was unrelated to the storde codes. It’s good that you found them. It’ll allow you to look into possible problems before they become significant.

P0300 is random mutiple misfores.
P0443 is a small EVAP system leak.

The former is likely something simple, like old sparkplugs.
The latter may have been a loose gas cap.