1995 Ford F-150 4.9L Engine P0133, P0153, P0171, P0174 Codes: Causes and Fixes with Crankcase Leak Diagnosis

If the vehicle has been poorly maintained for years with the check engine light on continuously, it's worth considering whether a full replacement might be more practical than continued repairs. A truck with over 100,000 miles and persistent issues may become a long-term financial burden. However, given its use for daily work (a 10-mile round trip), maintaining the current vehicle could still be viable if properly addressed.

Thanks for the input. The truck runs well and performs adequately under normal conditions—especially for a short commute. I believe it's more cost-effective to invest in maintenance and repairs than to buy another used vehicle with unknown history. My main concern is whether the oil leak from the lifter cover could be causing the lean mixture codes (P0171, P0174). If so, repairing or replacing the cover would likely resolve several issues.

If oil is leaking from the crankcase cover, it can allow unmetered air to enter the engine, resulting in a lean fuel mixture. The Powertrain Control Module (PCM) attempts to compensate by enriching the mixture, but oxygen sensors still detect the lean condition and cannot adjust properly—leading to O2 sensor fault codes like P0133 and P0153. The first step should be repairing or sealing the leak. Once that’s done, retest for codes.

The lifter cover is indeed a key component. If it has a significant crack, it can allow oil to escape and create an air leak—effectively mimicking a missing oil cap. This would cause lean conditions and trigger P0171/P0174 codes. While minor leaks may not always be noticeable, consistent air ingress will degrade engine performance and fuel efficiency.

Thanks for the helpful insight! After a closer inspection, I confirmed that what appeared to be a crack was actually just dirt buildup. The lifter cover was leaking oil, but after installing a new gasket, the leak stopped. I then replaced several worn components: mass air flow (MAF) sensor, spark plugs, distributor cap, rotor, ignition coil, PVC valve, and wiring harness. I also performed an oil change. After resetting the PCM, the check engine light stayed off for about 15 miles before returning. A follow-up scan revealed P0133 and P0171 codes. The tune-up improved performance noticeably. However, the truck occasionally acts as if several cylinders are missing when starting—especially cold or hot—though no backfiring occurs. I’ve already replaced the fuel filter within the past 18 months. The persistent codes suggest issues with oxygen sensors or fuel delivery systems. Given that this truck has dual fuel tanks, any additional advice would be appreciated.

You may still have a vacuum leak from the intake manifold gaskets—even if you’ve cleaned the area. If you're not ready to pay for professional diagnostics, consider purchasing new oxygen sensors directly from a Ford dealership and resetting the PCM. This can improve performance and fuel economy, even if it's only a temporary fix until the root cause is identified.

I'm back with an update: I replaced both remaining codes—P0133 and P0171. After disconnecting the battery for approximately 16 hours (a common method to clear O2 sensor memory), all codes disappeared. After driving about 60 miles, I attempted a vehicle inspection, which passed successfully. However, shortly after exiting the station, the check engine light reappeared. I returned to Auto Zone and found only P0171 present—P0133 was gone. I inspected for vacuum leaks and identified several minor ones. My question: Could multiple small vacuum leaks collectively cause a lean condition leading to P0171? And what should the expected fuel pressure be under normal conditions? Thanks for your help.

Yes, any vacuum leak—especially around the intake manifold gasket or PCV hose at the throttle body base—can cause a lean condition and trigger P0171. I recommend checking these areas thoroughly. Fuel pressure specifications for the 1995 Ford 4.9L engine: idle range of 45–60 psi, with a maximum of 50–60 psi under load. These values are critical to ensure proper fuel delivery and mixture control.

You've cleared the memory and passed inspection—but you haven't addressed any underlying issues. The real question is: do you want to fix the truck or just pass the inspection? You’ve not ruled out vacuum leaks, fuel pressure problems, MAF sensor failure, or oxygen sensor degradation. A full diagnostic approach is needed before assuming a simple fix will resolve everything.

I plan to fully repair and maintain the truck. Due to shift work, I don’t have consistent time for deep diagnostics. I truly appreciate all the advice provided. It's surprising that the inspection passed—my expectation was that P0171 would persist. After driving it for a few days without the check engine light on, I decided to test it again. Next week, I’ll spend dedicated time inspecting for vacuum leaks and measuring fuel pressure. I’ve already replaced the MAF sensor and checked intake manifold areas—no visible leaks were found, but I will re-inspect carefully. Also, the oxygen sensors were replaced about 18 months ago; I don’t know where they were purchased, but I’ll verify that next week. Thanks again!

Yes, only P0171 remains—no O2 sensor codes (P0133). I don't know the origin of the oxygen sensors; my brother-in-law replaced them about 18 months ago. I will confirm their source next week. I still need to conduct a thorough inspection for vacuum leaks and verify fuel pump pressure. I’ll share results once I have them.

So only P0171 remains—no O2 sensor codes? Where were the oxygen sensors purchased originally?

Yes, no O2 sensor codes (P0133) remain. Only P0171 is present. I don’t know where the original oxygen sensors were bought—my brother-in-law installed them. I will confirm their source next week. I still need to thoroughly inspect for vacuum leaks and verify fuel pressure. I’ll update everyone on my findings.