My Check Engine Light Came On...

A customer came in because their check engine light was on, the local auto parts store checked it and told them they have a bad gas cap. The trouble code on the printout was P0440, after replacing the gas cap and resetting the system the light came back on. The customer came to the dealership based on the assumption that they needed an original gas cap to fix the problem.

First off I would like to start by explaining that the scan tool can only determine the system that shows a malfunction and not pinpoint the actual failed part. More often than not it still requires testing and troubleshooting to find the actual cause of the problem.

Using the code P0440 as our example this indicates that a part of the EVAP control system is no longer functioning correctly. The EVAP system consists of many parts, including (but not limited to) the gas cap, fuel lines, carbon canister, purge valve, and other hoses. The EVAP emission control system prevents the escape of fuel vapors from a vehicle fuel system. Fuel vapors are routed by hoses to a charcoal canister for storage. Later, when the engine is running a purge control valve opens allowing intake vacuum to siphon the fuel vapors into the engine.

You likely won't notice any drivability problems and some of the possible problems are:

• The gas cap is not installed or working properly
• The purge solenoid has failed
• The canister is plugged and not working properly
• The line for one of the components are damaged

Here are some things to try:

• Tighten the gas cap, clear the code, and drive for a few days and see if the codes come back
• Inspect the EVAP system for cuts/holes in tubes/hoses
• Inspect for damaged or disconnected hoses around the Evap purge solenoid
• Check and/or replace the sensor
• Check and/or replace the purge valve

If none of these resolve the problem it may be necessary to have a professional use a smoke machine to detect leaks that could not be visibly found.

Why does my diesel need exhaust fluid?

Q. What is Diesel Exhaust Fluid (DEF)?

A. DEF is the reactant necessary for the functionality of the SCR system. It is a carefully blended aqueous urea solution of 32.5% high purity urea and 67.5% deionized water.

Q. What is Selective Catalytic Reduction (SCR)?

A. SCR is a technology that uses a urea based diesel exhaust fluid (DEF) and a catalytic converter to significantly reduce oxides of nitrogen (NOx) emissions. SCR is the leading technology being used to reduce deisel emissions.

Q. What is urea?

A. Urea is a compound of nitrogen that turns to ammonia when heated. It is used in a variety of industries, including as a fertilizer in agriculture.

Q. How will I know that the DEF product I purchase will work?

A. The DEF you purchase should state and display the certification of the American Petroleum Institute (API), German Institute of Standardization DIN70700, the International Organization for Standardization ISO 22241-1, and meet AUS – 32 specifications. This will ensure the proper purity and concentration (32.5%) of urea.

Q. How can I determine how much DEF fluid my vehicle will use?

A. The following website link can help estimate your vehicles usage:   

http://www.fleetguard.com/html/en/products/fuel/diesel_emission/def_calculator.html