2002 chrysler 300m error codes p0740 and p0700 what do they mean and is it a serious problem?

700 tells you that there has been a "transmission control circuit malfunction", and 740 basically pinpoints the possible culprit, which is "Torque converter clutch control circuit malfunction". If you don't notice any difference in the way the transmission is shifting, especially when you are at cruising speed on highway, or you've basically reached the speed your going to travel at, and are no longer accelerating, your converter normally goes into "lock-up" and your engine RPM's will drop by approx, 200-300 RPM's. When your driving it basically feels like transmission shifted again, slightly. If all feels well, it may just be a one time thing, but being how transmissions are somewhat temperamental, as well as expensive to repair when big problems occur, I would certainly consider getting it in to a dealer for diagnosis at least, and see what they come up with, as it may only be a bad switch, or bad, corroded connection causing the codes. Remember, if it sounds like an expensive,extensive repair, then simply get all the info, and either stop elsewhere for diagnosis of same problem, (without telling them it's already been diagnosed) and see what second opinion has to say. If in doubt after that, get back to us, with diagnosis results, and we'll be glad to help you decide from there. One thing you may want to consider though, is a transmission service, (oil and filter change) if it hasn't been done in a while, just as a preventative maintenance thing. Changing of oil and filter in those transmissions at regular intervals, is a good way of extending the life of them. I recommend the new ,Autopar ATF4 transmission oil, which is more expensive, but guaranteed to last for lifetime under normal driving conditions. Chrysler/Autopar products tend to work better with it, and new filter, although there are many mechanics that will argue that fact, there are just as many mechanics and owners who have seen other oils cause problems, and when they switch to Autopar, those problems disappeared.