2004 Nissan Sentra starting and idling issues

Yup. Diagnostic fault codes never say to replace parts or that they're bad. They only indicate the circuit or system that needs further diagnosis. When a sensor is referenced in a code, that sensor is the cause of it only about half of the time. You also have to look for cut or grounded wires and corroded or stretched connector terminals.

You didn't say what code was set. There are multiple codes that refer to an incorrect voltage on one of the wires, and there's different codes that refer to a missing signal, even though the circuit is okay electrically. That's where incorrect spacing comes in.

By backing the sensor out, it won't generate a signal and the Engine Computer might default to the crankshaft position sensor as a backup and try to run on that.

P0340 - Camshaft Position Sensor Circuit Malfunction

Unfortunately that code isn't very descriptive, but it suggests it's not an electrical problem. About all you can do is measure the voltages on that sensor but you have to do that with it plugged in by back-probing through the rubber seals. Typically you can expect to find 5.0 volts on the feed wire, and 0.2 volts on the ground wire. Beyond that you need a scanner to view live data to see if the signal is present.

Very often cam and crank sensor codes don't set just from cranking the engine. They set when the engine is coasting to a stop during stalling. You have to work with the code you read before any service was done or the battery was disconnected to erase the code. That's why it's important to read and record them before anything is done that will erase that memory.

The only way to test the computer is with very expensive specialized equipment that only the rebuilders have. For the rest of us, all we can do is to be sure we have all the 12 volt supplies, all the grounds, and all the proper inputs. If we have those and the computer is good, we'll get the proper outputs, ... Provided nothing is shorted on those circuits. There's often four 12 volt feeds and four ground wires. If you're missing the 5.0 volts to the cam sensor, that circuit feeds a number of other sensors, and any one of them could be shorted. To find that you'd have to unplug them, then turn the ignition switch off and back on to reset the power supply. Once you get the voltage back, monitor it as you plug each sensor back in. You'll set a lot of fault codes but it will identify which one is killing the 5.0 volts.

Don't concern yourself with the fuel system or starting fluid. It's the no-spark you need to address. Very often cam and crank sensor codes will only set when the stalled engine is coasting to a stop. Once it is erased, which also happens when the computer is unplugged to replace it, the code is erased and may not set again just from cranking the engine. You would need a scanner to view live data and see if a signal is listed as missing.