Perhaps knowing the purpose of the safeties would help. Note it is never a safe practice to jumper a safety, use your meter and see if it's open or closed and move on from there.
The pressure switch proves that the inducer is moving enough air through the heat exchanger and the flue is not blocked(at least not blocked before it ties in with a water heater). On a 90% or better AFUE a plugged condensate will back water up into the inducer where it will not spin fast enough to close a pressure switch even though the flue is clear. A plugged nipple on the hose going to the pressure switch can also cause a pressure switch not to close,even though the inducer is moving enough air through the exchanger it can't sense it.
Flame sensors take AC voltage from the control board and rectify it to a DC currant as the voltage jumps through the flame and returns to the control board via the burners and ground. Oxidation or sulfur from the gas put a fine coating on the flame sensor and act as an insulator for that < 1 microamp of currant. Rust on the face of the burners also act as a insulator for that currant.
Limit switches are safeties designed to trip if not enough air is moving across the outside of the heat exchanger. Dirty evap coils,dirty filters, motor failures,weak capacitors,control boards not sending voltage to the motors or not enough ducting are common causes of tripped limits.
Roll out switches trip when the flames or heat from the flames come out the front of the heat exchanger instead of passing through it. The most common cause is a serious breach in the exchanger. When a roll out switch trips it's time to put down the tools and call a pro in for a heat exchanger inspection.
On the subject of heat exchangers...They are steel and separate the byproducts of combustion from the air you breath. There are no safeties to shut down the furnace when it gets a crack, and they all crack or rust out at some point in time. The roll out will trip when the crack turns into a massive breach, or the exchanger gets full of soot from improper combustion, and flames belch out the front of the furnace. At that time you will get red tagged and have no heat until the furnace or heat exchanger is replaced so it's in your best interest to get the heat exchanger inspected every year(before the cold weather sets in).
