Reset the Check Gas Cap Message

When your 300 shows CHECK GAS CAP in the odometer window, it’s almost never a failed sensor. After a fill-up the powertrain module runs an evaporative-emissions self-test, and if it detects a leak — nearly always a cap that wasn’t clicked tight — it posts the message. It is a two-trip fault, so it neither lights nor clears instantly: you fix the seal and let the car re-test itself. No scan tool is needed.

How you know it’s the cap

The message on the 300 typically appears a few miles after a refuel, not the instant you drive off, because the monitor waits for the right conditions before it checks. On cars with the EVIC trip computer you may also see it paired with a chime. Crucially, the Check Engine light can join it a drive or two later if the leak persists — but a genuinely loose cap usually shows only the odometer text and no rough running, which is how you tell a seal problem from an engine fault.

1. Switch off and remove the fuel cap. Check the rubber seal for grit, fuel varnish or splits.
2. Refit the cap and turn it until it clicks at least once, giving a proper airtight seal. Fit a new cap if the old one is cracked or the ratchet is worn.
3. Drive normally. The message clears on its own after about three good drive cycles.

Why it takes a few drives to clear

The EVAP monitor only runs on a cold start with a part-full tank and mild ambient temperatures, so it may skip several trips before it finds a window to re-test. Once it runs and sees a sealed system, it clears the stored two-trip fault and the message drops out — which is why a single short hop to the shops often isn’t enough. Give it roughly three unhurried drives before deciding the reset failed.

If the message keeps coming back

A known-good, properly clicked cap that still triggers the warning points to a leak deeper in the 300’s EVAP system — the purge solenoid, the NVLD assembly, or a cracked vapour hose. A common owner mistake is fitting a cheap aftermarket cap whose seal never fully seats, so try a genuine Mopar cap before condemning anything else. If it still returns, the car has a real stored code worth reading: autodtcs.com walks through the P0440-series and P0457 EVAP codes and what each one points to. Tighten the cap with the engine off and the car parked, and if the Check Engine light starts flashing rather than glowing steady, stop driving hard and have it looked at.