Pontiac Repair: 02 sensors - how many?, air fuel mixture, intake manifold gasket

Question
Now I'm really confused! I've got a 2000 Pontiac Montana ...A/C, PS, towing package.

About 6 months ago I was getting a P0155 DTC, so I ordered the rear O2 sensor and installed it - easy! BIG performance change!

Now I'm getting P0131 and P0137 ...I thought that there were only two sensors on this vehicle: 1) on exhaust manifold (I can see it with a mirror between engine and firewall and 2) just after the Cat Converter (the sensor I replaced).

I took it to repair shop, told them I couldn't reach that forward sensor - please replace. When I picked it up, got sob story that BOTH Bank 1 O2 sensors (one at $175 and the other $199 + labor ...sheesh, the rear one I replaced onlycost $32!) had been replaced! Shop says I have a minor coolant leak (possible for this engine - 70K miles although haven't seen fluid level receding yet) that "probably wiped out BOTH the sensors on this side" - said they had to replace 2 of the four sensors and of course wanted to do the Intake Manifold Gasket job for another $900, which I declined, for now.

So ...how many O2 sensors ARE THERE on this thing? The shop manual I purchased when van was new only referenced two - the ones I mentioned earlier, both in location diagrams and schematics. So what kind of B.S. am I getting from these guys, if any?


Answer
Hi
There are only 2, the front one (pre converter) AKA Bank 1 #1 is an input to help fine tune air/fuel mixture.
Teh second one (post converter) AKA Bank 1 #2 is mainly a catalyst monitoring, to make sure the Cayalytic converter is doing it's job (OBDII).
Not wanting to slam or stick up for anyone....I have seen MANY times a coolant leak (contamination to a HO2 sensor)damage one.
HOWEVER if you are not experiencing any loss of coolant....
enough said.
My advise, take it o another shop, maybe another
recommended by friends, relatives or coworkers, and have them preform a coolant system pressure test to see if there is an INTERNAL coolant leak.
BTW the intake gaskets are VERY VERY common leakers on these engines.
I Hope this helps.
Good Luck
Pawl