Will not start, codes P0122 and P0622

I've only seen that once, but it turns out to be a somewhat common problem, but not on your engine. The 2.0L engine used in Neons is an "interference" engine, meaning if the timing belt breaks, any open valves will be hit and bent by the pistons as they coast to a stop. If the timing belt jumps one tooth, the Computer will turn on the Check Engine light and set a fault code for "cam and crank sync". If it jumps a second tooth, the computer shuts the engine down to protect the valves. If it jumps three teeth during repeated attempts to get it running, valves will be bent.

The problem these ran into was on only the single-camshaft engine, the dowel pin between the camshaft and sprocket would shear off, then over time, the sprocket would rotate slightly on the camshaft. That made the camshaft late, same as if the timing belt had jumped, but in this case it could end up the same as if the belt had jumped one and a half teeth. Since the cam sensor is on the other end of the camshaft, it looks at the camshaft, while we look at the timing mark on the sprocket. That's what makes this hard to figure out. All the computer knows is the camshaft is late, but it doesn't know why. That puts it right between allowing the engine to run with the Check Engine light on, and not allowing it to run. The computer turned the engine off by turning off the automatic shutdown relay. The one I had was off the equivalent of one and a half teeth, and the symptom was the ASD relay was switching rapidly on and off while cranking the engine. You'd get a few sparks intermittently out of only one of the two ignition coils, but the engine never started or ran for even a fraction of a second. Once the broken dowel pin was replaced, the engine started instantly with no need to relearn anything.

Your engine uses a timing chain and two camshafts, but it also has a hydraulic chain tensioner that might allow the chain to jump a tooth or two. There's no history of a broken dowel pin, and your engine may not even use that design. That means if a camshaft is late, you would be able to see that with the timing marks.

You're correct; there should be 0.0 volts on the 5.0-volt feed wires with the ignition switch off. There's two things I can think of that have often sent me down the wrong diagnostic path. One is there could be capacitors in the computer that need a little time to discharge. Think of them as very tiny batteries. Their water analogy equivalent would be like a tumor or balloon stuck on the side of the garden hose, like a wood tick. If you get little ripples, or changes in water pressure, that balloon will inflate and deflate slightly to absorb those pulses and make the pressure more steady. Capacitors can be used for multiple purposes, but in regulated power supplies, they charge up, in this case to 5.0 volts, and try to hold it there even if little voltage fluctuations appear. Once the system is turned off, those capacitors discharge relatively slowly. How long it takes depends on what the current can find to flow through, and how big the capacitor is. I've never checked this myself, but it could be entirely possible for it to take anywhere from a fraction of a second to maybe more than half a minute. During that time, you can still measure that voltage. In fact, if you leave the meter connected, you may be able to watch that voltage slowly go to 0.0 volts.

I have over a dozen digital voltmeters for tv, car radio, and auto repair, and all of them are of the manual type, meaning you have to select the scale, or range of what is being measured. A friend has an "auto-ranging" meter that selects the best range by itself. Each measurement can take a few seconds for it to find the right range, then display the reading. I don't have enough time left in my life to be waiting for that when I'm taking a dozen readings very quickly. The second thing you might have run into is what I got caught with multiple times when using that friend's meter. It could display "5", for example. I would glance over and see that as 5.0 volts, but when that doesn't seem right, after looking closer, I failed to see it had set itself to the "millivolt" range. It was really showing five millivolts from stray magnetic interference the meter's leads had picked up. I've seen people come to the wrong conclusion and even order the wrong replacement parts after overlooking the range on auto-ranging meters.

There's a third thing I can think of that you might have run into, but this too, I have never checked myself. Beginning with the '97 Intrepids, and maybe sooner on other models, the Engine Computer can take up to 20 minutes to go to "sleep" mode after the engine is stopped. Until that happens, it can draw up to three amps. Various emissions-related tests take place during that time that can't be performed with the engine running. It would stand to reason if the computer is staying powered up, every part of it would continue working, including that 5.0-volt power supply. You may have found 5.0 volts on the crankshaft position sensor while the computer was still awake, then found 0.0 volts on the camshaft position sensor right after it went to sleep mode. If you wanted to recheck that, all you have to do to wake the computer up is to disconnect the battery for a fraction of a second, or cycle the ignition switch to "run", and then back to "off".

Dandy. Happy to hear you solved this. Consider giving me an update in a couple of weeks to insure everything is okay.