P0173

P0173 is set when the fuel trim on bank 2 falls outside its allowable correction window in either direction — it's the bank-2 equivalent of an out-of-range trim flag and only appears on V-engines or horizontally-opposed engines with two cylinder banks. Unlike P0174 (specifically lean bank 2) or P0175 (specifically rich bank 2), P0173 often gets logged on vehicles whose ECU strategy reports a generic bank-2 trim malfunction without committing to a direction, so step one is always to read live LTFT and STFT on bank 2 with the engine warm at idle to find out whether the system is pushing positive (lean — adding fuel, look for vacuum leaks and MAF issues) or negative (rich — pulling fuel, look for leaking injectors or stuck purge valves). Once direction is known, the diagnostic collapses into the same workflow as P0171/P0174 (lean) or P0172/P0175 (rich): smoke-test the intake first for cracks in the bank-2 intake runner gaskets or boots, check MAF g/sec against spec for the engine size, then fuel-pressure test at the rail (35-55 psi port-injection, much higher for direct-injection). The single highest-yield 10-minute check is comparing bank-1 trims to bank-2 trims — if bank 1 is at +3% and bank 2 is at +22%, the fault is mechanical on bank 2 (bank-specific vacuum leak, clogged bank-2 injector, exhaust leak ahead of the bank-2 upstream O2 sensor) and not a global fuel-delivery problem.

Vehicle-specific patterns: 2003-2009 Toyota 4Runner / Tundra / GX470 with the 2UZ-FE 4.7L V8 commonly throws P0173 from a cracked rear PCV hose on bank 2 that's tucked under the intake plenum and nearly invisible without removing the intake — smoke test reveals it instantly. 2008-2014 Cadillac CTS / SRX with the 3.6L LFX/LLT V6 throws P0173 along with P0174 from a known intake-manifold sealing issue at the bank-2 side (GM bulletin coverage on cam-cover-to-manifold gaskets). 2004-2010 Ford Expedition / F-150 with the 5.4L 3-valve Triton throws P0173 frequently from broken intake-manifold runner control valves on bank 2 — the plastic shaft snaps and the runners flap open. 2000-2007 Mercedes M112/M113 V6/V8 throws P0173 from cracked intake-manifold air-pump check valves that introduce a one-sided vacuum leak. Repair cost typically lands $75-$500 — vacuum-leak fixes at the low end, manifold-gasket or IMRC repairs at the high end.