First thing that I can tell you is that your gas mileage must suck. Or more accurately, your car is sucking gas. Apparently, it is running rich (more fuel in the air than proper) all the time.

25/25 test causes the dyno to load 1/4 available hp at the wheels, based on the car's factory rating, at 25 mph. 50/15 uses 1/2 the hp at 15 mph.

So... at light load, the mixture is excessively rich, possibly below 10:1 where it should be 14.7:1 (air:fuel). At medium load, it is even richer. The car appears to be running out of the range that would happen in simple open-loop (uncontrolled by the O2 sensor) mode, even if thesensor were unplugged. The more likely failures here are coolant temp sensor (it is not the same one for the dash gauge), simulating an older car with the choke stuck partly closed. Or, a leaky diaphragm in the fuel pressure regulator. The low speed enrichment could be from the higher load, or from a constant fuel leak (bad FPR) having more effect at lower speed.

Pull the vacuum hose from the FPR and inspect for liquid at the fitting - there should be none. If it is good, a resistance test of the coolant temp sensor would prove if it is out of range. And this assumes that the coolant is up to temp of 185-195 deg F., which may not be the case. Your dash gauge should be rock solid on the 1/2 mark but a thermometer in the fluid is the only sure way to tell.

The high HC follows with the rich mixture. The cooling effect of the excessive fuel during combustion is probably causing artificially low NOx readings. You did not provide those numbers but I'd bet they are far below limits. As most have learned after EM test failures - this is the most likely gas to be high when the O2 sensor is the only failure.

EGR valve? Low NOx would indicate a working system HOWEVER, unless your car was built for California, it doesn't even have one.

Short version: either a bad coolant temp sensor or a proper reading of insufficiently heated (bad/missing thermostat) coolant is telling the engine to run as not fully warmed, or, fuel is leaking into the intake after the injectors and beyond the control of the injection system.

After fixing the problem and driving a few miles to clear some of the deposits, you may find that either NOx now fails, indicating bad O2 sensor - or all three gasses are slightly high, indicating a failed catalyst. It may be less costly to find a mechanic with equipment to check the car in tune-up mode after repair before going for state retest.

Leave a note afterward to let others know what really caused the trouble.