Hello and thanks in advance to all that will help

The car was running great, the gas tank was very low, so we stopped to fill up the tank and then right after leaving the station the car lost about 30% power but kept running, at first I thought it was bad gas so ran it on the highway for a couple miles but that did not help. I parked it in the garage and tried to start it the next day and it would turn over but not start, while cranking it seemed to backfire into the intake just a little but not out of the exhaust. I checked the gas for water but after pumping about 10 quarts out of the injector rail feed hose I did not find any water in the gas, fuel flow seemed good, did not check fuel pressure, plugs were wet with gas, and there is good spark at the 3 cylinders I checked. I checked what I think is the engine speed sensor (3 wires Blue, Gray, Black) and it had about 800 ohms across 2 and 3 and nothing to 1, also Tach does register about 300 RPM while cranking so I think it is good. I checked codes and found P0411- Secondary Air Injection System Incorrect Flow Detected which was there before this no start problem and P0422 - main catalyst below threshold bank (bank 1) which could be as a result of the no start e.g. too rich of a mixture past the OX sensor. I read on some posts that the gas vapor control system valve can cause this so I pinched off both hoses but still no luck.
Can the Crank position sensor be the cause? If so can it be checked with a digital meter? Is this a 2 wire Black connector (Blue/white and White/gray wires) on the driver’s side firewall?
I forgot to mention that I also pulled the timing belt covers and everything looks good there as well, no missing teeth, and with #3 at TDC the cam holes are on a horizontal plane. Yes the timing mark on the harmonic balancer was at TDC.
2 codes, " I checked codes and found P0411- Secondary Air Injection System Incorrect Flow Detected which was there before this no start problem and P0422 - main catalyst below threshold bank (bank 1) which could be as a result of the no start e.g. too rich of a mixture going past the OX sensor."
I agree that it sounds like a timing belt due to the intake backfires, however, the timing looks perfect and the belt is good.

Next I am going to take some gas from the non starting A4 and put it in my snowblower to see if the gas is bad for some reason other than water, and I am going to get a can of ether and see if it wants to fire with that (yes I will be careful and not use much:-), I also plan to hook a timing light to cyl #3 and check the electrical timing for spark at TDC of #3 and also have marked the cam gear and will see if the spark happens when the cam was marked (holes horizontal).

BTW, what is the relay marked 373 in the ECM box for? it does not seem to click in, I pulled it and tested it and it is good.

!!!!!!Any ideas would be greatly appreciated as I have now spent hours/days on this problem.!!!!!

 

Problem solved, it was really bad gas, wouldn't even run the snowblower. I just cracked a cold one to celebrate.