I took my 2008 sentra in to get an oil change and because me check engine light was on. I get a call the next day telling me that I have sludge in my engine, it is undrivable, and need a new engine. There is 35,000 miles on the car. And I can't prove when my last oil change was. I am unsure if I should take the dealership for what they say....any suggestions?

The alternator has nothing to do with engine sludge, but the engine sludge is the source of your issues. The oil light was not telling you the level was low, it was telling you that you had no oil pressure due to the sludge plugging up the sump and starving the engine of oil. The entire time you ran the engine with the light on you were killing it. Long story short, you either need the engine rebuilt or replaced with a good one.

have a compression test done and look for a moving valve seat insert
it gives the knock, misfire
as it is no 2 cylinder it is mechanical because you have moved the injectors and coils , plugs and leads and it stays at no 2 cylinder
If you want clean engine oil then change the synthetic oil out and put in diesel oil 20w40
It will not harm the engine as it is the same as petrol oil but with more additives than petrol oil
it is cheaper and has heavy detergent properties that keeps the engine internals clean
from the gunk in the sump ,it is clear that changing engine oil on a regular basis is something you don't do but if you want a reliable long lasting engine , it is must do number 1

There are two main causes for sludge, not changing the oil and filter at regular times, and not running the engine long enough to get it hot.
You might want to get a second opinion from a shop other than the dealer. The dealer may have a sales promotion on engine flushes this month.

I doubt you have sludge in your engine since oil has detergent in it to prevent this. I would stay away from this dealer.

First dealer was NOT doing you any justice. Next oil change run a can of GUNK engine flush let idle 5-10 mins then change Oil & Filter and use a good oil like Castrol or Valvoline 10w-30. You won't have "supposed" sludge build up.

run some diesel fuel thru the engine when u do the next oil change also try some high detergent motor oil or synthetic oil

These lights need to be reset manually. Try turn the key to ON (do not start) then pump the gas pedal 3 times.

Well i advice you to flush the engine,then put a good type of engine oil,after having service it,take out the positive cable from the battery for one hour ,this will reset the E.C.U .After one hour put the cable,then start the engine see what it gives.Hope that help you and solve the problem

It is doubtful that your engine needs to be replaced. Pistons don't fire, the spark plug that screws into the cylinder is what fires. Sludge may well be building up in your engine but I seriously doubt there could be that much sludge as to stall your engine at a toll booth...especially on a 2008...as long as you have done regular oil changes on the car.

Don't go back to that dealer, they're trying to rip you off. Take your car to a reputable mechanic and have them look at it. Don't tell them what the dealer told you...only tell them the check engine light is on. They will check the codes your car is throwing and then determine what is needed.

I repeat, DO NOT GO BACK TO THAT DEALER.

if this is an automatic there will be a dip stick if it is a manual you do not use transmission fluid to find out what sencor is bad take car to advance auto parts they have a digatal reader and can tell wou what sencer went bad and where it is located