Why would my check engine light go off by iteslf? I have a 2006 Kia Rio. Three weeks ago the check engine light came on with code P2096 and P0300, 0301 and 0302. Mis fire in two cylynders and oxygen sensor. I had a tune up, replaced the heated oxygen sensor and cleaned the fuel injectors. They said that would probably take care of both codes. A week later the same codes were back. They cleared them. The light came back on two days ago so I took it back. They said there was only one code this time and it ws P2096. They said mabey the heated oxygen sensor was defective and was still under warranty with Autozone. They didn't have time to fix it until next week. They cleared the code. Yesterday I drove 100 miles one way and the light came back on. By the time I drove the 100 miles back the code had cleared itself. Why would it go off by itself? I read that it takes time for the computer to get the message that the problem has been fixed. Could this be what happened? Should I take it in or wait to see if it comes back on? Thanks, Shirley

Dave C

• 1050 Answers
• Posted on Apr 19, 2011

SOURCE: I HAVE A 2005 KIA RIO THE CHECK ENGINE LIGHT CAME

Take it to another shop and have them do a compression check. If the compression is low, you have found the problem. If the compression is the same as the other cylinders, trade the fuel injector at that cylinder with another cylinder and see if the problem moves. If the problem moves (misfire code in different hole), the injector is bad. If the problem didn't move, and the compression is fine, then you have a vacuum leak on the intake runner to that cylinder, possibly the intake gasket. Print this and give it to your new mechanic.

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

SOURCE: Check engine light came on last week. They said

Yes a misfire could cause a false 02 code. As far as why it's misfiring could be a plug and wire issue but hard to say without looking at it