Why isn't power steering working?

Did you get all the air out (there is a bleeding procedure) of the system, sounds like an you have air in the fluid.

If all is assembled correctly and the system is bled by working the steering from lock-to-lock several times with the engine running and topping up the reservoir as required, the power steering should work. A pressure test would prove whether the pump is working correctly.

On later CAN-bus controlled cars the electronics might need telling the steering has been repaired or replaced before it will work satisfactorily.

could be the alternator or the power steering bearings. more like to be the power steering pump if it is the alternator spray some wd40 on the bearing shaft as the engine is running can do this to the power steering pump but it dont usually get to the bearing. unless you spray it in the breather hole on the shaft housing..

may be you should make a diagnostic and you can do it in a garage or you can do it by yourself if you want to do it by yourself you need some tools and you can find some good ones here: xcardiag.com

go to an accredited power steering specialist shop and have the pump replaced or have the shaft seal replaced on your pump

Have it checked that the power steering fluid is enough. If so, could just be that the system needs bleeding.

These usually have timing belts, if the water pump isn't making nose, leaking or dragging, put it back on. If it does have a timing chain in it, put new timing gears and guides in it as well, easier to do while you already have it apart. Check your power steering pump to make sure it is not leaking or the bearing isn't dragging, if so replace. Replace the thermostat while you have it apart. Check all hoses and belts to see if they need to be replaced while your at it, you will have most of them off anyway. Definitely check the serpentine belt that runs everything on the front of the motor, if it is cracked or worn, replace. Better to do it now rather than along the side of the road. Hope this helps you.

never replace parts until u know the solution-ull end up on valium--hook a tech ll scanner that runs a data stream to get the rite direction--cud be just bad sensors or batt connections or plug wires etc-sometimes just basic maintenance

Well, to start with, there are a whole lot of things (including a bad fuel pump) that can cause a cylinder misfire. A misfrire code does not mean that you automatically start replacing ignition coils.
The fuel pump is not supposed to have power to it any time the ignition switch is on. It will only have power for about 2 seconds to "prime" the injectors, then it will remain off until the engine starts and gets over 500 RPM.
Did you test any of the ignition coils to see if they were in working condition before you replaced them?
Did you test the ECU relay to see if it was functioning properly before you replaced it?
If the mechanic was able to scan the ECU and get codes ot of it, the ecu relay HAD TO be working at the time...WHY did you replace it?

ECU failures are extremely rare - especially on Nissans. However, poking around in the engine control circuits without knowing what you are poking on is a REAL GOOD way to GET an ECU failure!

I recommend that you read the following article and get back to me if you need further assistance:

What Else Could Be Wrong?

Have you checked the relays? Fuses? Possibly a short in the wiring or clean all connections in any harnesses related to.. Have you had someone run the obd coses on this? sorry to hear it. its like having a child and never knowing when they are going to fall down.
Do you have the chiltons or haynes manual. Always a good investment