02 sentra just replaces my engine with a used one and install 3 weeks ago it eat a lot of oil 1 can a week and my exhaust fume color blue.

You may have a broken ring or scored cylinder wall and piston. Have a cylinder leak down test performed to confirm this. Other causes can be a plugged crankcase ventilation system causing pressure to build in the engine, if that is the case the air cleaner box should have oil in it. The grey color of the smoke means you are burning a lot of oil.

the only way you lose oil is by burning it or leaking it, if your car blows a lot of blue smoke, then you are burning it {engine rebuild or replace} if there is oil on the ground when you park, then you need to determine where the leak is coming from, pressure wash underneath before taking it to a mechanic, we hate getting under dripping dirty cars. glad to help.

Take it to the dealership if it's under 60k miles. If it's sucking oil, then it sounds like you might have a bad head gasket, which is covered under the warranty. There's been a lot of hinky issues with the 2009 Sentra and #3 cylinder misfires, but apparently they don't want to do a recall on them. It's almost as if they're betting on the problem developing after the warranty expires.

possibly the pcv but check if your cars exhaust smoke is blueish or white in color if it does and your car doesnt have an oil leak then sure its burning engine oil along with fuel, the piston rings is defective, since you replaced engine oil the additive that the owner has put with the oil to prevent blowby is gone thats why it starts burning oil.....

A V8 engine will start to consume more oil as they get more and more
miles on them. The valve stem seals and piston rings wear, and let
more and more oil burn off in the exhaust. How many miles are you
travelling in said 3 weeks?

The main thing you will need to do is to minimise the amount of 'excessive' engine fumes being generated in your engine compartment which in turn will 'leak' into your car compartment: 1) Degrease your engine and check for leaking oil gaskets especially around the oil refill area: oil leaks at the back of the engine falling on your hot exhaust pipes and on your engine block will generate lots and lots of 'nasty' smelling fumes! 2) Check that your exhaust pipes joints are well sealed especially the one contained within the engine compartment: taking great precautions, check for the obvious leaks from smoke emitted but you could use soapy water to assist. 3) Check that the lining in your engine compartment are in good condition taking particular attention to the 'Firewall' ones (e.g. the one between the engine compartment and the passenger side compartment). 4) Check for broken heating/cooling air hoses leading into the passenger compartment.
Hope it helps!

First you need to determine if the van is "burning" oil or "leaking" oil. If the exhaust is bluish in color, this is a good indicator that you are burning oil and you probably need a valve job. A common issue with vehicles this age is an oil leak around the valve cover gasket. Oil leaks and burns off on the manifold. If when you open the hood and smell a great deal of oil, this is more likely the cause of using so much oil. The best way is to inspect the engine compartment first. If you can't spot any excess oil there, get under the van and look for a leak near the oil pan, oil plug, or more forward toward the engine compartment. Hope you find this answer helpful. Best of luck.
Greg

The blue smoke and oil consumption is because of the bad rings. They are allowing oil to get by into the cylinders and it's being burned with the gas. If you don't want to put the money into an engine rebuild, you can try one of the many additives available at auto parts stores that claim to help restore compression and reduce oil burning in worn or high-mileage engines.

check engine light has nothing to do with oil level. Only the red engine light does. It sounds like you are burning a lot of oil and it is poisoning your catalytic converter. Your converter is killing itself trying to treat all the unburned oil that is coming to it. It will get so hot that it swells inside and clogs it up. Why you are burning oil is the question. Only 3 possibilities. Past the piston rings, past the valve stem seals or a plugged pvc valve and its hoses. Clean the PVC hose and change the PCV first. Changing the cats now will just kill them again until you fix the oil burning.
Autozone will do free scan test for fault codes by the way.
PCV won't show up there though.
You can buy cheap engine on ebay or at car-part.com. page with asterisk is lowest price.

The oil pressure dropping to zero is common for cherokees. Replace the oil pressure sending unit and it will be good as new. Cracked exhaust manifolds are also common. Usually it will crack where the 2 pipes are welded into 1 pipe before it connects to the front pipe. There are lots of replacement headers out there to choose from. I just ordered one through quadratec.