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P0457 "loose fuel cap" error code


Feb 4, 2019
I’m getting a P0457 error (loose gas cap) on my 2005 LJ.

I’ve replaced the gas cap with an OEM Mopar one but that didn’t help.

Any other things I could try before bringing it to my local shop that don’t require dropping the tank?

Thank you



Staff Member
Sep 28, 2015
Salem, Oregon
Often times this is a leak in the evap system. The best way to find it (since it can usually be very hard to spot little leaks with the naked eye) is a smoke machine.
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Jerry Bransford

TJ Guru
Supporting Member
Nov 9, 2015
Escondido California
You may not have enough start cycles to clear the code. Have you cleared the code with the reader? If you have and it comes back it's probably a cracked hose or fitting below the filler cap like one of the various hoses or connectors around the gas tank.

Cisco Kid

Been around the block...twice!
Supporting Member
I had a Dodge Dakota that did that. After several gas caps I took them all to the shop. the checked it and they smoked it and could not fix it. No leaks.
What I did to stop the problem...Took the original Cap and tightened it but stopped before it clicked. Problem solved. Not a problem for two years.
Just tighten it and stop before it clicks. If it clicks then remove the cap and re-tighten it with a little less pressure.


Feb 4, 2019
So do I run the smoke machine along the Evap line and if there is vapor I’d see it in the smoke? Not sure I understand what to do with the smoke machine?


TJ student
Supporting Member
Here's a DIY smoke machine. It's a HF hand transfer pump for <$10.
Attach it to the intake as shown. Stuff a rag in the exhaust pipe and stopper the throttle body intake.
Light the cigar and pump smoke in and watch for it to show smoke somewhere.

Works the same on a 2.5L or 4.0L.