98 chevy venture: bank one lean

[dvdrose18]

starting at the begining.
A few weeks ago, my dad started getting a cylinder 1 misfire on his van. He knew the plugs and wires had ~60k miles on them, so we assumed it was them, and replaced them. this fixed the problem for a day or two, then the problem came back. At the same time it started to overheat at city speeds. Took it to a local repair shop which found a visable coolant leak aroung the intake gasket. so he brought it home and replaced the intake gasket. After replacing the intake gasket, he goes to get gas, and it starts overheating again. So he decides it is probably the head gaskets going as well, as the exhaust smelled of coolant. so, tear the engine apart, get the heads planed, (there was an obvious leak) and replace the head gaskets. puts it all back together and backs it out of the driveway with no problems. Goes to get gas again (never made it last time) and takes it out on the highway to see how it does, and New problem: when he lets off the gas, it stalls, and the computer is now throwing the code: bank one lean.
Any suggestions/ideas of where to look? could it be as simple as an O2 sensor that is not connected right?
Just trying to get some ideas of where to start diagnosing.

[tbaxleyjr]

Simply this means the O2 sensor servicing the #1 bank is reading lean. (I could go into more details but don't want to complicate the issue)

One needs to check the O2 sensor response (test procedures can be found in many shop manuals or Haynes tech books) and verify the O2 sensor is good and giving accurate information.

Once you verify the information is accurate from the O2 sensor one must look for the cause of more air entering the combustion process then the ECM can trim out. Usually, the culprit is a vacuum leak of some sort downstream of the throttle body (vacuum hoses, leaking intake manifolds, fuel injector O-rings, etc). Unfortunately, there is no magic bullet - you are going to have to hunt for the leak and fix. One way to do this is to squirt oil or carb cleaner at the gaskets and suspected leak areas while the car is idling and listen for changes in the idle characteristics. Your shop manual can provide additional details.

[dvdrose18]

After searching high and low for a vacuum leak and finding none, we decided it was the upstream O2 sensor. It was just about impossible to get it out as it is right up against the fire wall. Ended up rolling the engine forward and reaching up from below, but it was still terribly cramped getting it out. Seems to have fixed the problem, for now at least. Does it make sense that between the leaking head and intake gaskets and the cylinder misfire that we just totally fouled the sensor?

[tbaxleyjr]

yes - O2 sensors can be fouled