Odd code 176

Ok so the car is a 95 with explorer intake bbk CAI under drive pulleys and MSD ignition with full exhaust. The town I am stationed in is about 4100 feet above sea level. When I drive to the next town over about 40 miles or so into the trip there is a huge hill/pass with pretty rapid elevation gain. The car has to be in 4th gear for it because of the crappy 2.73 gears and 65mph speed limit. It just won't pull it in 5th.
I ALWAYS seem to get a CEL when I go up this hill. the only other time I seem to get it is sometimes at 80+ mph on the highway (legal btw). Alot fo times it will turn back off but sometimes will come right back on while maintaining speed. I never get it around town or anything. And it only appears to be the left bank. This is the driver side correct? No other codes really except a bogud gode 564 I think is the one for the fan circuit failure but the fan functions fine. The code 176 is coming from the KOEO test haven't ran KOER test. Ignition timing is about 14* with premium 91 octane fuel. Oxygen sensors are new. Going to try and tighten header bolts today since they have heat cycled a bit. Any other ideas?
 
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You could swap the O2 sensors and see if the code follows the O2 (i.e. the code moves to the passenger bank).
If so, just replace the O2 sensor.
 
Whoops, my apologies - I missed that you had replaced both O2's. So you had the same code with the old O2's and now two new ones? If so, I see your point and it's likely not the O2 sensor itself.

Given the intermittence of the issue, I like your next line of thinking. Beyond that, I'd almost wanna try to watch that O2's voltage while going up the big hill to see how bad it pegs on a low voltage output. If you do this, disconnected or dead O2's generally stick at 0.45-0.55V (you should get a code for non-switching). For a functional O2, the lower the voltage, the leaner the reading.

Were the pins in the connector's shell nice and clean? Those connectors can get covered in oil.