2005 Ford Expedition P0174 and P0304: Lean Condition and Cylinder 4 Misfire Diagnosis

This is an engine-related diagnostic issue — Firestone is primarily a tire retailer, though they do offer auto body services. For accurate diagnosis and repairs, you should seek out a certified automotive technician. To find the right shop: ask trusted friends, family members, coworkers, or church contacts for referrals. Visit potential shops in person to assess their cleanliness, equipment, and professionalism — this helps avoid being overcharged or misled. The P0174 code indicates a lean mixture in Bank 2 (driver's side), while P0304 points to a misfire in Cylinder #4. A lean condition can often cause misfires, so these issues may be related. A basic diagnostic session should take under one hour and typically involves checking common components such as vacuum hoses — especially the PCV hose — and verifying spark plug performance and fuel injector pulsing. These are straightforward fixes for this model year Ford Expedition. Note: P0174 on a 2005 Ford Expedition is extremely common, and in most cases, it's due to an intake manifold gasket leak (99% of the time). Any vacuum leak affecting only one bank will trigger this code; if both banks show lean conditions, then a more widespread leak exists. The P0304 misfire occurs on Bank 1, which covers cylinders 1–6 — so it's not directly related to the intake gasket but could be linked to ignition or fuel delivery issues in that cylinder.

Thanks for the clarification. I'm new to this area and haven't had a chance to visit any local auto repair shops yet. I understand Firestone offers some auto services, but I want to ensure I’m choosing a qualified technician rather than just getting a generic quote.

On 2005 Ford Expedition models, when only one bank shows a lean condition — such as P0174 (Bank 2) or P0171 (Bank 1 on the passenger side) — it's almost always due to an intake manifold gasket leak. Any vacuum leak affecting just one side of the engine will trigger this code. If both banks show lean conditions, then a more extensive vacuum leak is present. The P0304 misfire occurs in Bank 1 and affects cylinders at the rear end of the engine — it's not directly tied to the intake gasket but may stem from ignition or fuel delivery problems in Cylinder #4. A key warning: If you choose a national chain like Firestone, Dobbs, or NTB, be cautious. These shops often sell unnecessary parts and services to generate profit. Many employees are underpaid and lack proper training, which can lead to overcharging or recommending unnecessary repairs. Always verify the diagnosis before agreeing to any work — this is especially important for budget-conscious consumers.

Also consider checking the EGR valve — a clogged or malfunctioning EGR valve can cause lean conditions and contribute to misfires, particularly in older Ford engines like the 2005 Expedition. Cleaning or replacing it may resolve both issues.