Rattle sound underneath CEL comes on and off and white smoke coming from the exhaust

The thermostat is the most logical suspect for the engine not getting hot enough. The next step is to read and record the diagnostic fault codes. The people at many auto parts stores will do that for you for free, but be aware, those codes never say to replace a part or that one is bad. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition.

If the Check Engine light turns off at times while driving, it is very unlikely a fault code is being set relating to a failed catalytic converter. It is also unlikely more than one failed at a time.

First of all, forget the "mechanic-in-a-can" to clean the fuel lines. The cheapest, crappiest gas you can buy has all kinds of detergents and other additives in it already. Adding more of the same stuff isn't going to fix anything.

Dash gauges are notoriously inaccurate. Their purpose is for you to observe when something abnormal is occurring. I've had cars with temperature gauges that never went higher than one quarter but the heaters would burn you out of the car. I've also had cars with gauges that always read about 5/8, but the air from the heater wasn't especially hot. If you want to know exactly at what temperature the coolant is at, you need a scanner to view live data to see what the Engine Computer is seeing. There are also some data readers on eBay for well under $100.00. I bought one for a friend, and while it does an excellent job of showing the data, the screen only updates once about every four or five seconds. Regular scanners update their screens multiple times per second, but they cost many thousands of dollars too.

As for the diagnostic fault codes, it is standard procedure to read and record them, then to erase them and see which ones set again. Those that set right away are the most important and should be addressed first, but a lot of uninformed owners don't understand that those other codes will likely set too in the near future. That means another Check Engine light and the need for another diagnosis and repair bill. Most people think the mechanic didn't fix the problem right the first time, but in reality, there were multiple problems and the repair job was broken up into two parts. The customer has to return a second time for the second part of the repair job. Often that second part can be less expensive because some of the fault codes were the result of other things not working, that were fixed on the first visit. For example, the Engine Computer switches to "closed loop", meaning it adds the oxygen sensor readings to its fuel metering fine tuning calculations, when the engine reaches normal operating temperature. If the thermostat is opening too soon, the engine coolant won't get hot enough for the computer to go into closed loop. During "open loop" operation, the computer calculates a little too much fuel to insure no hesitation or stumbling problems occur during warm-up. A constant "too-rich" exhaust gas mixture is reported. Most of that extra fuel burns in the catalytic converters, but when a little sneaks through, the rear oxygen sensors will never switch to a "lean" condition. To the Engine Computer that looks like the converters aren't doing their job, hence the multiple "catalytic converter efficiency" fault codes.

This is only an example of what could happen, but the proper way to handle this would be to replace the thermostat and to erase the existing fault codes. The owner needs to be told that this COULD solve the converter efficiency codes and he should wait to see if additional repairs are needed. Too many customers are just sent on their way with the incorrect expectation their car is fixed. When fault codes set again and the Check Engine light turns on again, they no longer trust that mechanic.

White smoke from the tail pipe is a sign of burning engine coolant due to a leaking cylinder head gasket. Be aware that one of the byproducts of a properly-working catalytic converter is water vapor. It is common to see water dripping from the tail pipe or from a drain hole at the rear of a muffler. Suspect a leaking head gasket if you're losing coolant and have to add periodically. You can also add a small bottle of dark purple dye to the coolant, then check a day or two later with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. If the head gasket is leaking, you'll find the dye inside the tail pipe. Auto parts stores will have the correct dye for the fluid being tested, and those that rent or borrow tools should have a black light.

You don't have to replace all four wheels when one gets bent. You don't have to replace all the light bulbs when one burns out. And you don't have to replace all the catalytic converters because one rattles. The catalyst material in Ford's catalytic converters is a very porous rock. If a chunk of it breaks off, it can rattle. So what. If the noise bothers you, replace just that one. There have been times where I have been able to drill a small hole in the bottom of the housing, then run in a self-tapping screw, with the point ground off, to push the chunk up and hold it against the top of the housing so it didn't rattle. That doesn't work all the time, and the chunk may break apart further over time, then rattle again. That doesn't stop it from doing its job. If the exhaust gases were able to completely bypass the catalyst, that will be detected by the oxygen sensor after the converter, a fault code for "catalytic converter efficiency" will be set, and the code will specify which one.

Before you go looking for elusive problems that may not exist, replace the thermostat. You've described the symptoms perfectly multiple times as to what happens when it opens at too low a temperature. The dash gauge reads lower than normal, and the air from the heater is not hot enough. Once that is done, you'll find out if there are still other problems or they were caused by a failure of the engine to reach proper operating temperature.

First of all, please understand that just like doctors, carpenters, chefs, and accountants, mechanics speak their own language and usually have very poor communication skills when talking with car owners. For that reason, most shops employ a service writer who knows a little about cars, but is really good at taking what he thought he heard from the mechanic, and translating it into something he thinks you will understand. You know things are going to get lost in translation, but that is not an intent to defraud. A lot of shops, new-car dealers in particular, don't even want their mechanics talking with customers because they know too many things get mixed up. Also, mechanics have an interest in selling the job because that is how they earn their living. Service writers want happy customers to keep coming back. Their paycheck doesn't revolve around how much the customer spends. If they can save you a dollar without compromising the quality of the repair or the reliability of your car, they're going to do it.

The last thermostat I replaced was on my '88 Grand Caravan. It was easy to get to, and took about a half hour. Some vehicles are designed to get them off the assembly line as quickly as possible, with little concern to their serviceability later. That mostly pertains to GM products, but even on most of theirs the thermostat isn't that hard to replace.

I looked yours up, and the "flat rate" guide calls for 0.7 hours. That means every shop that uses that pricing method will bill you the same amount of time, multiplied by their hourly shop rate. The lowest hourly labor charge isn't always the best value. The higher-priced shops typically include little stuff like nuts and bolts at no extra charge. Often the less-expensive shops add every little item and chemical to the bill.

At the mileage you listed, a conscientious mechanic is going to recommend also replacing all or some of the hoses. The upper radiator hose will already be half removed, so it doesn't make sense to not replace it and hope it doesn't pop soon. He may also test the acidity of the coolant. Acids always build up in the coolant. All antifreeze has water pump lubricant, corrosion inhibitors, seal conditioners, detergents, and other additives, and those wear out in about two years. That is why the coolant must be replaced. GM owners know real well all about leaking heater cores and radiators due to the manufacturer's recommendation of not replacing the coolant often enough. They did that to make the cost of regular maintenance appear to be lower than that of their competitors. If it's time to have your coolant changed or flushed, this is the perfect time because part of that job is already being done. You'll pay for each job when done separately, or you'll pay a little less in total when related jobs are combined.

Also be aware that 0.7 hours is just for the thermostat procedure. That doesn't include any diagnostic time, time to drill out rusted and broken bolts, a test-drive or time required to warm the engine up to insure it's fixed, and things like that. Many shops bill this type of job strictly by the amount of time the mechanic is working on the car because they can't foresee the problems they might run into.