As was stated before, you gas cap will not cause a P0430, it is a completely different monitor than your Catalyst Efficiency. I posted some helpful information in an older thread regarding catalytic converters and diagnosing codes such as these. Instead of taking the time to rewrite the post I am just going to go ahead and pull it for you and post it here.
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Most if not all car manufacturers use the same testing parameters for catalytic converter efficiency, they will use a Pre-Cat O2 or AFR Sensor and a Post-Cat O2 sensor to measure the effective "oxygen storage" and from that it gives its result on what it thinks the catalytic converter's health is. That's it plain and simple, the ECU will look at Pre-Cat O2 levels and then Post-Cat O2 levels to determine if the catalytic converter is actually working. This presents a small plethora of possible problems and scenarios ranging from complete catalytic converter failure to biased O2 Sensors. The reality is that this is nothing more than a Go, No Go test, because the catalytic converter does not deal only in O2. The only proper and 100% accurate way to test a catalytic converter's efficiency is to preform an intrusive test, where you drill into the cat and use a 4 or 5 gas analyzer to verify functionality. But, most people don't want to drill into the cat and most customers don't like that idea either...
In any case, if one of these codes is tripped you should always assume an "innocent until proven guilty" frame of mind towards the problem. First test the O2 sensors individually and then compare to what the scantool is providing as information. After verifying that the sensors are working properly and that the scantool data is in-line with the sensors I would hook up a 4 or 5 gas analyzer to the exhaust and measure the gases. This won't give you a 100% accurate view of what is going on, but it will help you figure out what the catalytic converter is doing. Without going deeply into the operation and chemical reactions that occur inside of the catalytic converter, the gases will vary according to what is actually happening. For example, if your catalytic converter was working well there would be little Oxygen coming out the tailpipe because it is used in reaction to modify quantities of NOx, CO, CO2, and HC. Keep in mind also that the catalytic converter is NOT the problem, a catalytic converter should last the life of the vehicle or at the very least a whole hell of a lot longer than most people will keep the vehicle. If the vehicle is operating as it is supposed to your catalytic converter should not fail prematurely, this is a SYMPTOM of another problem, not a problem in it self. Something to keep in mind when dealing with these codes/repairs, because I have seen guys slap a set of sensors and cats on a car and have it come back with the same problem months down the line because the real problem trashed the cats again.
There are other things to consider, such as if the vehicle has been driven with faulty sensors for a long time sustained damage to the catalytic converter may have occurred and it will need to be replaced anyways. The sensors may have damaged the cat but it could be "brought back to life." I have seen catalytic converters be brought back to life before, but really its a 50/50 shot in the dark.
Here are some links:
HowStuffWorks "How Catalytic Converters Work"
http://www.converterwarehouse.com/te...rter-&News=100
Catalytic Converter Tech Tips : Catalytic Converter Replacement : ConverterWarehouse.com
All Catalytic Converters - How do I know if my catalytic converter is defective?
Hopeful this will help you out.
