P0171 Lean Code + 37 MPG Fixed – Fuel Pump/Filter Assembly Replacement (2011 Prius, 255k miles)

Hey everyone,

Just wanted to share a recent fix that took months of chasing but finally solved it. My 2011 Prius (255k miles) had been stuck around 37 MPG (vs typical 50, degraded over ~30K miles), threw P0171 “System Too Lean (Bank 1)”, and had LTFT consistently 30–40% at idle and driving. No hesitation, no power loss, no rough idle—just poor economy and the code.

What I tried (and what didn’t fix it):

• Gas cap tightened → No change
• MAF cleaning → P0101 cleared
• New air filter → No change
• New MAF sensor → P0101 gone permanently, P0171 still returns after ~60 miles
• Extensive brake/carb cleaner spray test (all reachable hoses, clamps, intake manifold top/sides, throttle body gasket, EVAP purge, etc.) → No RPM/LTFT change so no vacuum leak
• Thorough throttle body cleaning (plate both sides, bore, edges, intake entrance) → No change
• Two tanks with Techron Complete Fuel System Cleaner → Marginal mpg bump (36.8 → 37.4), no LTFT change

Live data notes:

• LTFT 30–40% across idle, 2500 RPM, maintenance mode, highway load
• Bank 1 Sensor 1 (wideband) ~3.3V steady lean
• Bank 1 Sensor 2 (downstream) normal ramp-up on cold soak
• No drivability issues despite the high trim

The fix: Took it to a local shop. Fuel pressure test showed ~30 PSI under all conditions (should be 44–50 PSI). Diagnosis: clogged in-tank fuel filter / weak pump assembly.

BTW - just so every knows: the Gen 3 Prius does NOT have a fuel pressure sensor nor a fuel-line Schrader valve.

Replaced the assembly, Part No.: 77020-47081, and new O-ring part number 77169-06060.
Prep: Removed rear bench seat, vacuumed/cleaned area, pried up cover with thin metal puddy knife, vacuumed/cleaned again, and soaked the metal retaining ring twice overnight with PB B'laster (focusing straw under the lip).

Used the Orion Motor Tech Fuel Pump Removal Tool (adjustable 4-jaw) with 18" breaker bar. Only two of four jaws fit securely due to the metal ring’s design (Part No.: 77144-4704), but with slow steady pressure it popped off without issue.

Pulled out old fuel pump/filter assembly and installed new one.

After clearing codes, mixed highway/city test drive → LTFT dropped to 2–5% (max 9% WOT), instant 49.4–50.1 MPG on a 66-mile test run.

Replaced the cover using 3M Strip Calk, 08578.

I used the Panlong OBD2 Scanner with the apps Dr Prius and Car Scanner on an Android Pixel 6 phone to collect the engine codes and the sensor data.

Total DIY time: 74 minutes.

This DIY video was the best one for replacing the fuel pump/filter assembly:

I've attached a few images of the cover, ring, tool and close up of the fuel/filter assembly part number label.

Bottom line: If you’ve ruled out MAF, throttle, vacuum leaks, and O2 sensors, and LTFT is stubbornly high with poor mpg but no power loss—don’t overlook the in-tank pump/filter. It’s a common silent killer on high-mile Gen 3s. The hybrid motors mask any stumble, but the ECU pays the price in fuel trims and economy.

Hope this saves someone else the months of chasing. Thanks to the forum for all the threads that pointed me in this direction!