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98 Ford F-150 P0171/P0172/P0174 Diagnosis: Lean/Rich Codes Conflicting After Engine Rebuild

Model: 98 Ford F-150 Fault Code: P0171 Posted: 2007-04-30 12:54

My 1998 Ford F-150 with a 4.6L engine is still not running properly after a recent engine rebuild. The check engine light (MIL) was already on prior to the rebuild, and it continues to show persistent trouble codes: P0171 (Bank 1 lean), P0172 (Bank 1 rich), and P0174 (Bank 2 lean). The truck shakes noticeably at idle, especially when stopped at traffic lights or in park, but the shaking stops once I begin moving. I've already replaced all four oxygen (O2) sensors, performed a full tune-up with new spark plugs and wires, and cleaned the MAF sensor—none of these fixes have resolved the issue. A mechanic tested fuel pressure and vacuum lines, reporting lower-than-normal fuel pressure and normal vacuum levels. Based on this, I suspect a fuel delivery problem. I've already changed the fuel filter, but symptoms persist. This is especially urgent because in California, an active MIL triggers immediate smog test failure. I need a reliable diagnosis to get this truck passed for emissions. Any insights or troubleshooting steps would be greatly appreciated—especially regarding conflicting lean and rich codes that appear simultaneously. Thank you.

Related fault codes
P0171P0172P0174
Comments (9)
Anonymous 2007-04-30 14:15

Check for intake gasket leaks—this is a common cause of misfires and incorrect O2 sensor readings in older Ford F-150s. Also consider replacing the MAF sensor with a new one.

Anonymous 2007-04-30 15:48

The codes you're seeing are inconsistent: P0171 and P0174 indicate lean conditions, while P0172 shows a rich condition on Bank 1. It's highly unlikely for both banks to be running one way at the same time. Clear the codes, perform a road test, and report which codes return—this will help identify whether the issue is truly conflicting or due to sensor inaccuracies.

Anonymous 2007-04-30 17:19

P0171: Lean fuel trim on Bank 1; P0172: Rich fuel trim on Bank 1; P0174: Lean fuel trim on Bank 2. The presence of both lean and rich codes on the same bank is contradictory. This suggests a deeper issue—either an intake gasket leak causing unburned fuel, faulty injectors with poor spray patterns, or a malfunctioning O2 sensor (especially on Bank 1). A proper diagnostic using an OBD2 scanner and real-time data readings is essential to pinpoint the root cause. Consider either performing this at home or taking it to a shop for full analysis.

Anonymous 2007-05-01 09:55

Lean and rich issues can worsen over time if left unaddressed, especially when driving conditions are poor. As problems accumulate, diagnosing them becomes more complex—requiring a systematic approach with proper tools and data interpretation.

Anonymous 2007-05-01 11:35

Many mechanics rely on basic code readers that don't provide detailed insights into actual fuel trim or sensor behavior. The last mechanic I visited recommended replacing spark plugs and wires, but that didn’t fix the issue. I now suspect an intake manifold gasket leak may be contributing to the problem.

Anonymous 2007-05-01 11:47

Is the engine truly a new rebuild? If so, it's important to verify that all components—including injectors and seals—were properly installed during assembly.

Anonymous 2007-05-01 12:05

Prioritize resolving the fuel delivery issue as this is likely at the core of the problem. Poor fuel pressure or inconsistent fuel flow can directly cause conflicting O2 sensor readings.

Anonymous 2007-05-01 14:09

It's highly improbable that all four O2 sensors would fail simultaneously—especially without any other symptoms like engine misfires or rough idling. This suggests one or more of the sensors may be faulty, or there’s an underlying issue such as a vacuum leak or fuel delivery problem.

Anonymous 2007-05-01 16:36

While fuel delivery is a key factor, it's worth noting that the fuel pressure regulator's vacuum signal typically comes from a dedicated vacuum port—often connected to a runner on one bank or a central distribution point. A leak in this line could cause erratic fuel trim readings and explain conflicting lean/rich codes.