Let me start helping you sort this out by clearing up some misinformation. Code 420 does indeed refer to catalytic converter efficiency. The engine computer varies the fuel / air mixture between rich and lean about two times per second. The front oxygen sensor detects the extra unburned oxygen in the exhaust as a lean condition, and the lack of that oxygen as a rich condition. Therefore, it's "switching rate" is about two times per second.
The catalytic converter stores the unburned oxygen, then uses it to burn the unburned fuel. As a result, the gas leaving the converter is very different than what came into it. When the converter is working properly, the switching rate of the second, or "downstream" converter may be as low as once every minute or two.
As the converter begins to lose it efficiency, it fails to clean up and burn some of the hydrocarbons, so the downstream switching rate speeds up. Eventually the switching rate will match that of the front O2 sensor. That is what tells the computer to set code 420. The computer won't wait that long though. There is some magic switching rate number where the code sets once that threshold is crossed.
What is important to understand is for that code to set, there has to be exhaust gas flow, which means the engine has to be running. A stalled engine won't set that code. More importantly, the computer knows about this problem because it is interpreting the readings from, a properly-working sensor. The sensor is reporting the fuel / air ratio, so it is the mixture that is the problem, not the sensor. (Compare that to an overheating engine. You need to fix the cause of the overheating, not replace the coolant temperature sensor that simply reported the overheating).
With every one of the more than 2,000 potential fault codes, there is always a long list of conditions that must be met for that code to set. That list always includes a number of other codes that can't already be set. In this case there are dozens of potential fault codes related to oxygen sensors, and they mean very different things. Some of those codes refer to problems with the sensor itself. When one of those is set, the computer knows that sensor cannot be relied on to provide accurate information, so it will stop running any tests that use that sensor. That means if you have a code for a defect with the rear O2 sensor, you will never get a code for catalytic converter efficiency. The test for that is not running. To say that a different way, if you get a code for catalytic converter efficiency, there cannot be a code already set for either oxygen sensor, and both sensors have to be working.
The next red flag is you stated you are using premium gas, and you have a misfire. There should be a fault code telling which cylinder is misfiring. If the cause is spark-related, as in worn spark plugs and wires, the mixture in that cylinder will go unburned into the exhaust system where the gas will burn in the catalytic converter and overheat it. That will lead to the efficiency problem, from the catalyst becoming melted, and the code 420. The catalyst material melting greatly reduces its surface area that is supposed to momentarily absorb the oxygen and store it.
Using premium gas is going to increase the likelihood of a spark-related misfire. Many people incorrectly think premium gas produces more power or provides some other benefit. It absolutely does not. Now, if your owner's manual states that premium gas is required, that is a different story and you may disregard this paragraph. The majority of engines have a specific compression ratio that is conducive to providing the desired amount of power from the standard grade of gasoline, and the gasoline is formulated to do that job in those engines. For comparison purposes, most gas engines have compression ratios of around 8:1. Spark is needed to induce a temperature high enough to start the gas burning. Diesel engines have compression ratios of around 22:1 to 24:1. Squeezing the mixture that much causes its temperature to go up high enough for the fuel to ignite on its own without the need for spark. That high compression ratio gets us a lot more power from the fuel.
We can get more power from gas engines too that have higher compression ratios, but that approaches the point of where it wants to self-ignite, just like diesel fuel. We control when that occurs in diesel engines by carefully timing when the fuel is squirted into the hot air. In gas engines, the gas is already in the mixture when it is being compressed, so there is no way to control or adjust when it starts to burn. Instead, to avoid the resulting spark knock or pre-ignition, we keep the compression ratio low enough to avoid that, then rely on the precisely-timed spark to initiate the burning.
The point of this sad story is there are some applications where we want to get as much power as possible from the gasoline. We do that by increasing the compression ratio, but then we have to do something to the gas to prevent the pre-ignition. High-octane, (premium) gas has additives that make it harder to ignite to do this. We used to increase the amount of lead in the gas, but today we use cleaner and safer additives. Muscle car engines from the 1960's and 1970's had compression ratios around 10:1, and those needed premium gas. Race engines, even today, can have compression ratios of over 12:1. Premium gas won't even stop pre-ignition in those engines. They require aviation gas that has octane ratings over 100.
The point is, premium gas does not develop more horsepower. Using premium gas allows engine designers to design an engine that develops more power from gasoline. High-compression engines need the higher octane. Low-compression engines do not need higher octane and do not benefit from its use. Since higher octane gas is harder to ignite, you are much more likely to get a misfire from marginal spark plugs or anything else that results in weaker than normal spark. Stick with the lowest grade possible that does not cause spark knock, unless a higher grade is specified for your engine. Lower grade, or lower octane does not relate in any way to lower quality. Quality of fuels is more closely tied to the additives in it.
For the coolant loss issue, if you no longer see the results of an external leak, a leaking cylinder head gasket is the best suspect. There' are two ways to test for that. One is to perform a chemical test at the radiator with the engine running. Air is drawn through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present, the liquid will turn bright yellow. It is best to have your mechanic perform this test. It takes just a few minutes. You can usually borrow the tester from an auto parts store that rents or borrows tools, but they will make you buy your own bottle of fluid, which costs more than paying your mechanic to do the test. The fluid is rendered ineffective if it is allowed to freeze or if it is contaminated with antifreeze. That could lead to false results for the next person who uses the tool. That's why they make everyone buy their own bottle of fluid.
The second method, which works well for very slow, hard-to-find leaks. Is to add a small bottle of dark purple dye to the coolant, then search a day or two later with a black light. Auto parts stores have different dyes for different fluids, and those that borrow tools will have a black light. The dye will show up as a bright yellow stain that can be followed back to the source of the leak. If you have a leaking cylinder head gasket, you will find the dye inside the tail pipe.
Sunday, November 27th, 2016 AT 4:43 PM