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98 Ford F150 P0171 and P0174 Codes: Causes, Fixes, and Common Solutions

Model: 98 F150 Fault Code: P0171 Posted: 2006-12-30 11:14

I'm experiencing P0171 and P0174 diagnostic trouble codes on my 1998 Ford F150 with a 4.6L engine and 150,000 miles. I've already replaced both oxygen (O2) sensors, changed the spark plugs and ignition wires. The check engine light can be reset and stays off for extended periods, but returns when accelerating harder—especially on the freeway. Driving in town is generally fine. I also notice pinging under load, even after switching to higher-quality fuel. Could this be related to the EGR valve? Are there other potential causes I should investigate? Thanks, Kerwin

Related fault codes
P0171P0174
Comments (4)
Anonymous 2006-12-30 12:16

Common causes include EGR valve issues, DPFE sensor faults, low fuel pressure, vacuum leaks from intake gaskets, or a dirty MAF sensor. There are many possibilities—though I’d be surprised if it were the O2 sensors themselves. Could you confirm if this is something you’ve been told by someone else? This might help narrow down the diagnosis.

Anonymous 2006-12-30 16:10

I recently replaced my O2 sensors for a different code, and I did a full tune-up just last month. It turned out the plugs had never been changed before—this is common on trucks with over 150K miles. A local shop suggested the EGR valve might be at fault, but I haven’t yet replaced it.

Anonymous 2007-01-02 08:02

You mentioned replacing O2 sensors due to a different code—what was that code? This is relevant because 1998 Ford F150 models had a known technical service bulletin (TSB) related to P0171 and similar codes. The issue stemmed from cold condensation in the AC system dripping onto the HEGO sensor, causing thermal cycling. Symptoms included hesitation, stumbling, surging, misfires, rough idle, near-stall or stall at idle, and detonation during acceleration. If this sounds familiar to your experience, please let me know. Generally, P0171 and P0174 indicate either restricted or leaking air intake—this is a very common issue on these vehicles.

Anonymous 2007-01-02 17:25

Try cleaning the MAF sensor—it can become clogged over time. If you're using a K&N-style air filter, consider replacing it; oil buildup from the filter can gum up the MAF. Also inspect for vacuum leaks around the intake gaskets and PCV hoses—this area is one of the most common sources of such problems.