Why does the check engine light comes on saying the gas cap is loose or off when it is on tight.

SOURCE: the check engine light comes on my 2006 Kia Rio once a week. COde is P0496. I had the solenoid and gas cap replaced ands they said they can't find anything wrong with it. Why does it keep comin

First of all, it does absolutely no good to clear codes without properly diagnosing the code and correcting the problem that caused the code to set in the first place. It will only result in the code returning again and again as you can now see.

You must also know how the system works and what failure is indicated by the code. Please note that fault codes NEVER tell you what parts to replace. Then you must know what conditions in the system cause the code to set in order to perform a valid diagnosis of the system.
failure.

With all that said, let's take a look at the code itself:
Diagnostic Test Code (DTC) P0496 is defined by SAE J2011 as "Evaporative Emission System High Purge Flow".

Purge flow is determined by the Powertrain Control Module (PCM) by looking at the Fuel Tank Pressure (FTP) sensor. When there is little or no purge flow, the FTP sensor will read high pressure. If there is too much purge flow, the FTP will read low pressure.

The code is clearly stating that there is too much purge flow (or low pressure/high vacuum in the tank). The people that have been trying to diagnose this problem apparently do not understand this. Leaks in the system like leaking lines or defective fuel caps will cause the pressure to be too high when it is SUPPOSED to be low and will set a code P0455, P0456, or P0457. Smoke testing and fuel cap replacement is what you do for THESE codes, NOT code P0496.

In simple terms, what is happening here is that the PCM is telling the EVAP system to stop purging and it is continuing to purge anyway. This is usually caused by a stuck open Purge Control Solenoid Valve (PCSV). Now I do not know which solenoid they replaced, but I am willing to bet that they replaced the Canister Control Valve (CCV) also known as the "vent solenoid". Again, this might be a good strategy for the other codes listed above, but not the code we are working with.

The other thing that eats at my craw here is that they are randomly replacing parts, evidently without testing them first to see if they are defective and/or are the CAUSE of your problem. I do not call this diagnosis. I call this "parts-nosis". This is where a technician just starts replacing all the parts in the system untill he finally finds the one that takes care of the problem. And YOU have to pay for it! Not only are you paying for parts that do not fix the problem, but they also have the nerve to charge you for "diagnosis" time to find the problem. Then they wonder why people think that mechanics are nothing but a bunch of rip-offs!

So plain and simple, they are not finding anything wrong with your EVAP system because they are simply looking for the wrong problem. They are looking for a system leak when they should be looking at why the pressure is low or vacuum keeps increasing when the PCM is telling it to stop.

There is pretty much only three things that can cause this code:

> Stuck open PCSV (most likely)
> Defective fuel tank pressure sensor
> Defective or mis-calibrated PCM