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EVAP emissions system leak behind ‘check engine’ light

A faulty gas cap can cause an EVAP system leak code to register in your car's onboard computer.Your car’s “Check Engine” or “Service Engine Soon” light can flash on for more than 1,400 different reasons.

The light is part of your vehicle’s on-board diagnostics (OBD) system and can indicate anything from a quick fix like a loose gas cap to a major repair like a failing catalytic converter.

The purpose of the OBD system is to reduce tailpipe emissions, and your car is programmed to alert you any time the engine control computer detects that a sensor or system isn’t performing the way it’s supposed to.

Here’s another example of a problem that can trigger the light…

Trouble code: P0456 Evaporative Emissions System – Small Leak Detected

The EVAP system is a closed system that captures fuel tank vapors and prevents them from escaping into the atmosphere. When your engine light comes because of a weakness or failure in this system, you most likely won’t notice the car driving any differently. Everything will seem fine, except for the glowing check engine light on the dashboard.

Here are some possible causes:

  • Missing or loose fuel cap.
  • Incorrect fuel cap used.
  • Evaporative (evap) emission canister or fuel tank leaks.
  • Evaporative (evap) emission system hose leaking.
  • Fuel tank leaking.
  • A small leak in any of the EVAP hoses or fuel tank hoses.
  • A small leak in the purge valve or vent valve.

What to do if your ‘Check Engine’ Light comes on:

First, confirm it’s the ‘Service Engine’ or ‘Check Engine’ light. If you’re not sure, consult your owner’s manual. (Here’s our guide on how to respond when a dashboard warning light turns on.)

If it is the engine light, you won’t know whether it’s because of an EVAP failure or something else (unless you try to go through the emissions test lane or stop by an auto parts store that will retrieve the diagnostic trouble code or codes).

But what you can do first is see if the gas cap is to blame. Check the gas cap to make sure it’s tight and there’s no rust or anything else preventing it from sealing. If that all checks out, you’ll want to schedule an appointment with your auto repair shop for further testing.

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We’ll be able to retrieve the code or codes, which will tell us which system is having a problem. And from there, we can do pinpoint testing on the individual components within that system to get to the root cause.

For example, if you do have an EVAP emissions code, we would test for leaks in the fuel cap area, the fuel filler hose, the evap canister, your fuel tank, fuel lines, evap hoses, and the purge or vent valve. And then we could tell you exactly what’s happening, and more importantly, we can accurately estimate the cost of the repair.

Have questions about a check engine light or need to schedule an appointment? The team at Auto Lab is always here to help.

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