The P0174 code in your 2016 Toyota Tacoma indicates a lean fuel condition on bank 2 (the bank not containing cylinder 1 on V6 models). The sudden drop in fuel economy makes sense because the ECM is adding fuel trying to compensate for the lean condition, but the root cause means fuel is being wasted without improving combustion.
Understanding Bank 2 Lean
A lean condition means too little fuel for the amount of air entering the engine. When this occurs on bank 2 specifically, it suggests an issue localized to that side of the engine rather than a system-wide problem. The ECM detects this through the bank 2 oxygen sensor readings.
Why Fuel Economy Drops
In trying to correct the lean condition, the ECM adds fuel across the engine. This extra fuel doesn't burn efficiently because the underlying air issue remains. The result is wasted fuel, incomplete combustion, and potentially rich conditions that further hurt efficiency.
Common Causes
Vacuum leaks on the bank 2 side of the intake manifold. A failing bank 2 fuel injector delivering less fuel than commanded. A dirty or failing bank 2 oxygen sensor providing false lean readings. Exhaust leak before the bank 2 O2 sensor affecting readings. Fuel pressure issues affecting bank 2 injector performance.
Diagnostic Steps
Compare fuel trims between bank 1 and bank 2—significant difference confirms bank-specific issue. Inspect bank 2 intake and vacuum connections for leaks. Check oxygen sensor readings for bank 2. Test fuel injector performance on bank 2 cylinders. Check for exhaust leaks near the bank 2 catalytic converter.