Nissan needs new computer?

I have a 2000 Nissan Maxima with 80,000 miles and it won’t start. Its not the battery – it just won’t turn over. The mechanic I gave it to put in a new fuel injection register (?) for $50, and he says it still doesn’t work and that it is 90% likely it is a computer problem. He says this is a rare problem that happens to some cars. He found a computer to replace the old one at a junk yard for $200 and says the total estimate for all the work will be $500-600. Does this diagnosis and cost sound right?

“Turn Over” means the starter turns the engine over. “Starts” or “Fires” means the engine starts. If you mean the starter cranks but doesn’t start, and the problem is that there is no spark to the spark plugs or signal for the injectors, then yes it could be the computer. If you mean that the starter doesn’t crank the engine over, it has nothing to do with the computer. Ask what will happen if that doesn’t fix it? Have you checked fuses?

Some more info would help.
By not turning over I’m guessing here that you mean the starter motor will not physically crank the engine over.
If that is the case do you hear a starter solenoid “click” sound when you turn the key to the START position.

If not, then I would suspect either a neutral safety switch if the car has an auto transmission or a clutch safety switch if the car has a manual trans.
There is also a lesser possibility of a faulty ignition switch (the electrical part).
If the engine will crank over but not start then even more info would be needed to hazard a guess.

At this point I’m pretty skeptical of the computer diagnosis and don’t know what a fuel injection register (?) is.

I believe what the mechanic changed was a Fuel Pressure Regulator. I, further think, that you mean that the engine won’t start. I agree with the others that I don’t think the problem is the computer because they fail so seldom. On many cars, you can’t just change the engine computer, the computer has to be “introduced” to the car with a (usually dealer’s) scan tool.
Find a more believable mechanic. He hasn’t made me a believer.

I’m not sure the 2000 Maxima have that but might be a dropping resistor for injectors. (It comprises a circuit for adjusting a voltage and rarely fails)