I have a rookie question - when the car shows a service code, does it really have a sensor that reads the fluid being low or its just by milege or timeframe?

e.g. I know the oil is probably a sensor since it almost always comes on when it's 15% but what about transmission fluid or brake fluid or differential fluid?

 

Engine oil life is not determined by a sensor but by a calculation based on mileage, number of starts and stops, driving style, etc. The change intervals for other fluids are determined the same way.

 

Some items are determined by the vehicle owner:
- one year max on engine oil if you don't get the M.I.D. notification within 12 months
- brake fluid 3 year interval (newer Acura's have a code for brake fluids)
- spark plugs at 100,000 miles
- serpentine belt (usually done at timing belt service at 100,000 miles)
- Timing belt, code pops up a +/- 105,000 miles; but, many owners change out the timing belt in the 7-11 year time frame if they are under the 105k mileage (can include serpentine belt since that needs to be removed for TB)
- water pump, can done at Timing belt service to save on labor cost
- AT/transfer case can be done early without M.I.D. notification if you tow a lot (near max weight, hilly terrain, stop/go traffic, most travel includes towing, etc...)