Q: I have a1999 infiniti qx4 v6 . Trouble code p0180 comes up and p0180 pd. I replaced the sending unit. Still won't start.

The trouble code P0180 could be related to the no start condition if the fuel is frozen from extreme cold or if there is water in the fuel. Also, if the fuel gets too hot and vaporizes, that can cause the code to set and will result in no start, as well. Excessive additives in the fuel can cause erratic sensor readings and cause the fuel to boil off at lower temperatures than usual, causing the sensor to malfunction. You should be sure the electrical circuit to and from the temperature sensor does not have any loose or high resistance connections. If the sensor is new, the circuit has been verified (no voltage drops), and there is no issue with fuel temperature or quality (old gas with water, too, perhaps), then manually erase the code and attempt to start the vehicle. If the vehicle still does not start, the fault lies elsewhere.

Be sure there is adequate fuel in the fuel tank and that the fuel is not old. Check for adequate fuel pressure, injector operation, and a spark at the spark plugs. Basically, if the starter motor is turning the engine over rapidly, at sufficient RPM, but the engine does not catch and run, that means that there is an ignition, fuel, air induction, or mechanical fault in the engine that will have to be repaired. Strictly speaking, if there is fresh fuel being delivered by the injectors (not just "to" the injectors) and the ignition system is working, that means that the air fuel ratio of the mixture delivered to the cylinders is so far off that the mixture just won’t ignite. Such could be due to a big vacuum leak, an air induction fault like a stuck idle air control valve or, again considering fuel, could be due to a failed temperature sensor that must be working properly to signal the PCM to enrich the mixture on cold start. Other common faults are defective spark plugs or a defective coil(s). Regardless of the underlying cause, if you request a no start diagnostic the responding certified mechanic will get the problem diagnosed and repaired for you promptly. Please let us know if you have further concerns or questions as we are always here to help you.