Toyota Repair: 2004 Toyota Camry, oil, burning

Question
I have a 2004 Toyota Camry.  My first question is does my car have a timing belt or timing chain and how often does it need to be changed.  I've been to two different things.  Next, my car currently has approx. 225,000 miles because I drive about 100 miles to work Monday thru Friday to work, I've noticed that smoke comes from the exhaust after the car has been sitting for about 5 hours and overnight.  I've been told that I need valve seals and then asked if I was getting my oil changed regularly.  And that answer is yes, I was taking my car to the dearlership every 5,000 to 6,000 miles.  Next question, last summer the check engine light came on but I never found out what the code was because I was on my way out of town my mileage was around 150,000 miles, do you have any idea what the code may have been.  My girlfriend has the same car and she said hers indicated an O2 sensor, do you think mine may have meant the same thing.  And what happens when the sensor is not replaced and the seal is not replaced.
Thanks.......

Answer
Well that is a mouthful!
First question, you have a timing belt and it should be changed every 100,000 miles. (change the water pump at the same time, whether it needs it or not - because it will fail between belt changes if you don't.
White smoke out of your exhaust after sitting awhile (e.g. overnight) could simply be condensation in the exhaust, which is normal.  Does it continue to "smoke" after driving several miles?  Have you had to add oil between changes?  If you are not losing any oil, then it is NOT a problem.
Is the check engine light still on or has it burned out.  To see if it is burned out, turn the ignition switch to ON (not Start).  All your lights should come on (this is a self-test).  They will go off after several seconds so see if your "Check Engine" light illuminates.  If it does, then the light works.  If it doesn't, the light doesn't work and you could be having troubles and not know about it.  Next time you get your oil changed, have them check your codes and clear out any old ones.
You could have an O2 sensor failure, or a thousand other things wrong.  You need to find the code to aid in troubleshooting.  If it is the sensor, replace the seal also.

Hope this helps.

Vince