Volvo: 1995 Volvo 850, From Very Rough Idle to Now Wont Start This Morning, a2 code, oxygen sensor

Question
I took my 95 850 T-5R out for a 20 minute trip this morning.

When I attempted to restart the car a few minutes later, the car went into a very rough idle and eventually stalled.  I tried to restart the car a few more times and had the same result.  If I tried to put apply a little pressure on the gas pedal, the car would stall out.

Now the engine will turn but no longer start.  I ran a check of the diagnostic codes and come back with the following:

A2: 4-3-5
A3: 2-1-1, 2-1-2, 2-1-3, 2-1-4
A7: 1-2-2, 1-2-1
B2: 1-2-2
B5: 2-2-2


The A2 code (Front Oxygen Sensor Slow Response) had triggered a "Check Engine" light approximately 7 months ago.  I successfully cleared the code and did not see the Check Engine light come up for another 3-4 months (which I subsequently cleared).  During this whole time the gas mileage and engine performance did not seem abnormal.

However, the same light did come up this past Friday night after I came to a sudden stop when driving.  I successfully cleared the code on Saturday morning, and the car ran fine the entire weekend.

Is it possible that the front oxygen sensor could be causing this problem?

Here's the interesting thing.  When I clear the A2 code, I hear a sound from inside the engine that is best described as "a valve shutting off" or a "vacuum hatch closing".

There may be one last clue to the puzzle.

On Saturday night, going about 50 mph on the highway (it was raining and the roads were wet), I looked down and noticed the floor gearshift (automatic transmission) did not seem completely "clicked" into place.

So in an admittedly dumb move I slightly jiggled the gearshift to snap it into place and the gearshift accidently went into neutral for a split second (which I very quickly put back into drive).

Could this have anything to do with the car refusing to start?

Any advice would be greatly, greatly appreciated.

Warmest Regards,

Ron

Answer
Ron,
When ever there is a code for O2 sensors always replace both O2's with unit that are from the dealer. Not all O2 sensors are of the same quality or will work. Been through it and now use only Volvo ones.
If the car will not crank then I can see a problem with the gear shifter and gear position sensor. That sensor gets old and worn so it make poor contact and that can cause no cranking and codes in the transmission...