A sudden drop in fuel economy on your 2018 Nissan Sentra combined with code P0174 indicates the engine is running lean on bank 2. This lean condition causes the computer to add more fuel trying to compensate, resulting in decreased fuel efficiency along with potential driveability issues.
Understanding P0174
Code P0174 indicates the engine control module detects a lean condition (too much air or too little fuel) on bank 2. The computer attempts to compensate by adding fuel, but this compensation has limits. The added fuel to correct the lean condition reduces your fuel economy.
Why Lean Conditions Waste Fuel
It seems counterintuitive that too little fuel causes poor fuel economy. However, the lean mixture burns inefficiently, produces less power per combustion event, forces the computer to add extra fuel, may cause misfires that waste fuel, and makes you press the gas harder to maintain speed.
Common Causes of P0174
The Sentra's 1.8L engine can run lean from vacuum leaks at intake manifold or hoses, mass airflow sensor contamination or failure, clogged fuel filter restricting delivery, weak fuel pump unable to maintain pressure, exhaust leak before oxygen sensor, and stuck-open PCV valve.
Diagnostic Approach
Perform a smoke test to find vacuum leaks - the most common cause. Check MAF sensor for contamination and clean or replace. Test fuel pressure under load conditions. Inspect exhaust manifold for leaks before the sensor. Review fuel trim data to see how much compensation is occurring.
Repair and Results
Fixing the lean condition should restore normal fuel economy. Vacuum leak repair, MAF sensor service, or fuel system repair depending on cause. After repairs, fuel economy should improve within a tank or two as the computer readjusts fuel trims. Monitor to confirm improvement.