[JustSayGo]

First of all your Altima does have two O2 sensors. The rear O2 sensor is after the cat. You will find the front O2 sensor is located in the exhaust manifold. You have two circuits in the front O2 sensor with three wires. One is the heater circuit and the other sends a message to the ECU.

Speedometers are not sensors. The speedometer and odometer display the message that comes directly from the VSS (Vehicle Speed Sensor). The VSS would also affect an automatic transmission if you had one.

The tachometer measures engine RPM and uses the same signal from the CPS (crankshaft Position Sensor) that signals spark and fuel injection The CPS is located inside the distributor. The tachometer connection is made at the CPU because it is convieneint.

Your VSS is bad or the connection plug where it connects to the wire harness is not making a good connection. The VSS problem is also the cause of the speedometer-odometer problem. The VSS is in the transmission near the starter and the half-axle shaft.

If you failed emissions because of excessive Hydro-carbons, replace your 02 sensor. Remove and clean both battery cable connections and making sure they are tight when you are through. That should clear the codes and eliminate the cause possibly being from a bad ground. Check the ground cable between the engine and the body. Make sure the cable isn't loose or missing.

The VSS doesn't effect fuel mixture in a way that will make emissions excessive.

The transmission could be making so much noise that the knock sensor sends a signal to the CPU and spark timing is retarded but can't change the noise.

Bad fuel contaminates O2 sensors, but does not effect the knock sensor you will find in the engine block near the oil filter. I suspect that you may find that all three sensors are bad. All three are easy to replace once you are under the car. No doubt the VSS will test ok since the problem is intermittent. You will find the O2 sensor is contaminated from over 100,000 mi use. If you decide you need to buy the VSS, buy all three sensors online and combine shipping. You will save plenty.