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Seeing P0171 and P0174, a rocking idle, and a stumble off the line? On this 4.0L SOHC V6, aged injectors and dried O-rings are the usual cause. The fix is a matched set of six — the six-piece 0280156028 injector set for the Mercury Mountaineer and Ford Explorer 4.0L.

You pull the rail once either way. Six cost the same labor as one, and the other five have aged the same miles.

What P0171 and P0174 mean

Both codes mean the engine runs lean — more air than the fuel can match. Bank 1 and bank 2 are the two halves of the V6. The PCM (the engine computer) adds fuel until that correction hits about 20 to 25 percent, then stores the code.

Which banks set the code tells you where to look:

• One bank lean only → a leak on that side: one intake gasket runner or one injector O-ring.
• Both banks lean → something all six share: the fuel rail, the plenum gaskets, or six injectors aging together.

Three ways the injectors go lean

• Under-delivery. Varnish from ethanol fuel cakes the pintle and seat, so a 24 lb/hr injector acts like a smaller one. Both banks go lean.
• Dried O-rings. Each injector seals with two rubber rings. Hardened rings leak unmetered air, worst at idle — a known 4.0L SOHC habit.
• Poor spray. A worn pintle sprays droplets, not mist. That is the cold-idle shake that fades as the engine warms, often with no code.

All three respond to the same fix: fresh, matched injectors with new seals.

Which calibration your 4.0L actually uses

The tail letters are the trap: C5A versus B5A is one etched letter and a 20 percent flow gap. Reading your own injectors settles every one of these questions.

A 20 lb/hr set in a 24 lb/hr calibration drives the trims lean and re-stores your codes. For BA builds, the 0280156029 six-piece set for the Ranger, Explorer, and Mazda B4000 4.0L is the matched unit; for the 1998-2001 trucks, the 0280155962 six-pack for the 1998-2001 Ranger, Explorer, and B4000 4.0L.

Generic "fits everything" listings deserve skepticism here. Owner threads put the SOHC 4.0 at 24 lb/hr; the older OHV 4.0 used 19. Buy back the factory number. Want headroom for modifications instead? The performance injector ranking guide explains how flow sizing is chosen.

Year windows and the budget path

Catalog windows overlap on this engine, so the etched number outranks the model year. Match the number first; the year window only picks the listing. Inside one window, the year-specific listing costs less for the identical job: the six-piece set scoped to the 2002-2004 Mountaineer and 2002-2003 Explorer 4.0L.

Prove the cause before you spend

Injectors are the third suspect here, not the first. Two cheap checks keep the order honest:

• Smoke test the intake at idle. Smoke from the plenum gaskets, a PCV fitting, or an injector bore names the real leak. Ford's TSB 03-16-1 cites the intake gaskets; the cracked PCV hose is the other 2002-2005 habit.
• Read the trims against rpm. Lean at idle that improves as rpm rises → air leak. Lean that worsens under load → fuel delivery: pump, filter, injectors.

Once the air side passes, test the injectors:

1. Unplug each connector and measure the coil: about 14.5 Ω. An open circuit, or a reading far from its siblings, convicts that unit.
2. Run a balance or contribution test; the cylinder whose crank-speed drop differs owns the weak injector.
3. Try a pressure decay test. It catches the injector that weeps fuel into the port after shutdown and fouls a plug.

Injector sets, regulators, and throttle bodies live in the air and fuel delivery collection.

Renewing all six

This is a hand-tools job through the top of the engine, with no programming afterward. Work in order:

1. Relieve fuel pressure per the service manual and disconnect the battery negative.
2. Remove the upper plenum plumbing and rail fasteners; the rail lifts with the injectors in it.
3. Unplug and release each injector from the rail.
4. Transfer nothing from the old units; the set ships with its own O-rings. A reused hardened ring hands the leak right back.
5. Oil every new ring with clean engine oil and press each injector into the rail.
6. Seat the rail in one square motion so no sideways load hits the seals.
7. Cycle the key twice to prime, check every seal for leaks, then let it run.

The idle improves within minutes; the trims take longer. Fuel trim is learned memory, so it walks back to zero over a few drives. Near zero a week later means the repair holds.

Codes back on fresh injectors? Smoke the intake again — something else is feeding it air. Ford's V8 DTC logic is mapped in the Ford 4.6L/5.4L injector guide, and the 4.7L Dodge Dakota and Durango guide covers another six-pack truck where a matched set wins.

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