Q#1 My check engine light keeps coming on. I had the code checked and they say that it’s an evaporate leak and the gas cap caused it. I got a replacement gas cap and the light came back on. I had the code checked and it was the same code. They said it might be a defective gas cap so they gave me another one. The next day the check light came back on and it was the same code. They told me I would probably have to go to the dealer to have it diagnosed. The dealer wants $75 just to get the code and $100 an hour to diagnose the code. I told them that I already had the code and shouldn’t have to pay $75 for them to check it again. They told me that the tool and the software needed to diagnose the problem is very expensive and they have to charge for it. Why is it, the parts store does not charge anything to diagnose the check engine light, but the dealer wants $75 just to start? If the code is for a gas cap why doesn’t replacing it FIX it?
Not all scan tools are created equal
A#1 First off, there is a big difference in the cost and capabilities of the tools you’re talking about. Second, the parts store did not diagnose your vehicle’s computer. They only retrieved a code, a code that had a description with possible causes for the fault. The parts store tool is about as cheap as so called computer scan tools can get, that’s why it’s called a cheat-a-reader. You can buy one on Amazon for $12.69 with free shipping. It will read most computer codes, possibly provide a code description and check readiness monitors. The time it took you to read this paragraph is about as much as you can get out of one of these tools.
A code is not a diagnosis
As for diagnosis, just because you have a code doesn’t mean the problem has been diagnosed. The computer code is only reporting that there is a system problem.
An example would be the part store’s information. They only retrieved a code that had a description with possible causes for the fault, somehow a gas cap was part of that information. Clearly, the second gas cap was about as needed as the 1st. The parts store did not diagnose your vehicle’s computer. They only fetched a code from the computer.
What is an evaporative system?
The evaporative system is designed to prevent gas vapors from your vehicle leaking into the atmosphere. A missing, or loose gas cap that is not sealing correctly can turn on the check engine light with a code related to the “Evaporative Emission Control System”. As you look below you’ll see that there are 16 “Evaporative Emission Control System codes”, and 4 of these codes could be directly related to a gas cap problem, yet may have nothing to do with the gas cap. When the computer performs a system test it is making an assumption; it does not know that you live in New England. It doesn’t know we spread enough salt in the winter to give everyone on the planet high blood-pressure.
Here are a few examples:
- A rust hole in a gas filler tube can set the same code as a gas cap problem.
- A cracked rubber vapor hose can set the same code as a gas cap problem.
Here’s the boring list
- P0440 Evaporative Emission Control System Malfunction
- P0441 Evaporative Emission Control System Incorrect Purge flow
- P0442 Evaporative Emission Control System leak Detected (small leak)
- P0443 Evaporative Emission Control System Purge Control Valve circuit Malfunction
- P0444 Evaporative Emission Control System Purge Control Valve Circuit Open
- P0445 Evaporative Emission Control System Purge Control Valve Circuit Shorted
- P0446 Evaporative Emission Control System Vent Control Circuit Malfunction
- P0447 Evaporative Emission Control System Vent Control Circuit Open
- P0448 Evaporative Emission Control System Vent Control Circuit Shorted
- P0449 Evaporative Emission Control System Vent Valve/Solenoid Circuit Malfunction
- P0450 Evaporative Emission Control System Pressure Sensor Malfunction
- P0451 Evaporative Emission Control System Pressure Sensor Range/Performance
- P0452 Evaporative Emission Control System Pressure Sensor Low Input
- P0453 Evaporative Emission Control System Pressure Sensor High Input
- P0454 Evaporative Emission Control System Pressure Sensor Intermittent
- P0455 Evaporative Emission Control System Tank Detected (gross leak)
The difference in cost between the pro level scan tools can range from $2,000 to $10,000, even more depending on the application. Software updates range from $500 to $1500 annually depending on the manufacturer. This level of equipment allows you to talk to the vehicle’s computer and the technician to have bi-directional control of all kinds of things for diagnostic purposes. This control is necessary since many components on today’s vehicles are computer controlled, for example; windows, wipers, dash gauges and even the controls for the Evaporation Emission Control System, which is very necessary for diagnosis when it is not just a gas cap.
Another necessary piece of equipment is an evaporative system tester. This tool will actually pressurize the system from the gas cap to the motor and all of the hoses, valves, charcoal canister and more. It also introduces smoke which makes it easier to identify leaks, and for the really small leaks the smoke lights up especially well with a black light.
Even more, this machine also measures the leak rate of the evaporative system, which helps to locate a leak smaller than the ball on the end of a ball point pen. If you take a look, that’s pretty small…. and a leak even smaller will make a check engine light come on too!
This makes it a little easier to understand, when it is not as simple as a gas cap, a check engine light for an evaporative control system can require a substantial investment in tools and training to effectively diagnose the problem/cause.
The check engine light can come on for many many codes
Have you even wondered how many computer codes there are in today’s vehicles? Use the link below to another page that has a good list. This list does not include the latest additions for hybrid vehicles.
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