Hi there. I'm having issues with my M45. I'll keep it short.
130K miles, up to date on oil changes and fluids. Here's what my problems are.

SES Light on for 1 issue

-P0051: HO2S21 Sensor 1 Heater Bank 2

other issues:
- one side (passenger, right) of exhaust is completely cold, not pushing out exhaust.
- other side (driver, left) running very rich, on cold days water forms and drips constantly from left exhaust. May be head gasket but does not seem likely.

Other notes:
- only modifications are muffler delete (straight piped only where muffler is) and 10 Inch Sub with amp.
- Fuel economy down to 14 MPG, (should be 16.5 as stated by Infiniti)
- Drivability/performance does not seem to be affected as it is not sluggish, does not stutter on acceleration, does not have rough idle.

I am going to take it in to Infiniti to get diagnosed, however, I would like some insight. Is it safe to drive in this condition, although the car information system shown on the dash says it is? I have not been driving it due to these issues.
What are the possible issues I could be having, any of these (Catalytic converter? Oxygen sensor? Head gasket?)
Any help is greatly appreciated.

I can't believe no exhaust is moving and it still runs. Converter, and/or O2 sensor could be issue.

I think I figured out why you think there is no exhaust, stock these cars have a 2 into 1 then splits back into 2 exhaust, so if one of the "tips" is not pushing air, or dripping water it appears one side of the motor isn't. by the way the water drip isn't indicating running rich, that isn't gasoline dripping. in cold days the heat from the exhaust coupled with the natural humidity in the air causes the water to condense out.

MSeries61996 wrote: ↑

Wed Oct 10, 2018 11:11 pm

...
SES Light on for 1 issue
-P0051: HO2S21 Sensor 1 Heater Bank 2
...
What are the possible issues I could be having, any of these (Catalytic converter? Oxygen sensor? Head gasket?)...

I think you already have the answer. Replace the primary oxygen sensor and the code should go away along with your fuel economy trending up.

On safety, I wouldn't encourage you to drive the car extensively, especially if you have other options.

...
- one side (passenger, right) of exhaust is completely cold, not pushing out exhaust.
- other side (driver, left) running very rich, on cold days water forms and drips constantly from left exhaust. May be head gasket but does not seem likely.

Other notes:
- only modifications are muffler delete (straight piped only where muffler is) ...

When you say one side of exhaust is cold, where are you checking it - at the engine exhaust manifold, or at the tailpipe?

Along with that question, how many pipes do you have coming out of the bumper? 2 pipes feed into the muffler, into one box, then come out as 2 tips. If you directly connected into these 2 individual pipes when you did your muffler delete, then it's not surprising. The right side of the exhaust has a butterfly valve that opens when certain criteria are met.
So yes, if you have 2 separate tail pipes, then it is very feasible that the right (US passenger) side isn't getting a chance to open and heat,up.
Look at this thread as well as the others linked within it - y34-factory-exhaust-cutout-no-way-dual- ... 03328.html

I am going to guess you do short drives at moderately low speeds?
Water in itself is a product of combustion. It's going to be there all the time, but most times it comes out of the tailpipe as vapor so you might not see it. I don't know how cold the "cold days" in your location get, but it might take a while to get the entire exhaust pipe hot... Hence water vapor in the exhaust gases condensing and coming out as liquid.
That's all assuming when you said the "water forms on the exhaust" you're referring to the tailpipe.

If you have a concern with the head gaskets or the cooling system, check your coolant reservoir as well as the filler neck.
Coolant smells sweet btw.

hey Ed, I used to live in North Dakota, temps were so below 0 that -5 felt like a warm balmy day. in those type of temps water does drip, even forms icicles from the tailpipe

What was the outcome?

Turns out, only issue was a bad O2 sensor. Not sure why it didn't feel hot when it was running the code but as soon as I replaced the sensor itself, the right pipe was just as hot as the driver side and the butterfly valve for the "Dual-mode Muffler" would open under load or at about 4700RPM, as stated in the manual. It is also pushing out exhaust from the right pipe again. I'm fairly new to Nissan/Infiniti cars, and a young mechanic with most of my learning being hands on, so it was surprising for me when I read this and realized there was indeed a butterfly valve and it may have had some response to the O2 sensor itself.
The car runs smoothly again. This repair took place about 2 months ago, so I think she's doing alright. Sorry for the long wait on the response. Thanks everyone!
Now to find the ever evasive front struts

Also, no Check Engine codes!