2000 GMC Sierra 4.3L P0172 and P0175 Codes: Fuel Pressure Regulator Fix & O2 Sensor Issues

The most likely cause is a leaking fuel pressure regulator or fuel lines connecting to the upper intake plenum. You can access the upper intake without removing the distributor or lower intake. With the key cycled, use a flashlight to look for fuel pooling inside the intake. If you find fuel accumulation, it indicates a leak. In such cases, replacing the aftermarket fuel pressure regulator assembly is recommended. Good luck—let us know your findings!

I didn’t realize this truck has a dedicated fuel pressure regulator—I thought fuel pressure was set by the tank-mounted pump. How do I check it? And if the regulator leaks, would fuel pressure still read 60 psi? This model doesn't have a traditional fuel rail; instead, injectors are mounted directly on the upper intake. I’m not saying you’re wrong, but I'm unfamiliar with this setup. I’ll try using a flexible borescope through the throttle body opening with the butterfly fully open to inspect for fuel pooling. I’ll update everyone once I have results—thank you for the help!

Even with 60 psi, check how quickly pressure drops when the key is turned off—this should be rapid if there’s a leak. The fuel pressure regulator is located underneath the upper intake plenum. Refer to Haynes or Chilton manuals for diagrams, and watch videos on Wells Ignition or YouTube showing the injector setup and servicing procedures. If you use a borescope, it's helpful—but even then, if you block the return line and pressure still drops quickly, the leak is likely in the fuel lines leading to the regulator or injectors. In all cases, removing the upper intake is necessary for proper inspection.

That video covers a lot more than just injector maintenance—very useful for owners of Vortec engines.

Ah, I see now—the fuel pressure regulator is under the upper intake! I replaced the lower intake gaskets last spring and didn’t notice it before. The only component I recall was the fuel injector spider assembly on the lower intake. I don’t know if pressure drops after shutdown because my fuel gauge only shows max pressure until you press the release button, then drains a small tube attached to the regulator—but the truck starts immediately every time with no delay. I tried using a borescope but couldn’t angle it properly to view the back of the manifold. With the butterfly fully open and key cycled multiple times, I didn’t see fuel spraying or smell excessive fuel. I’ll try pinching the return line next time, but if the truck runs normally despite the codes, maybe there’s no major leak. Still, fuel efficiency has dropped noticeably.

After some online research on NAPA's site, I found that the fuel pressure regulator is bolted upside down at the base of the fuel injector assembly—very sneaky! I’ll definitely need to remove the upper intake housing and inspect it while the fuel pump is running.

Well, partially. After removing the upper intake and cycling the key, I didn’t see any fuel pooling near the regulator or injectors—checked each one individually. However, when I removed the fuel injection spider assembly, I found a layer of fine fibers or debris covering the small intake screen inside the fuel pressure regulator. Since I had it out, I replaced both the fuel pressure regulator and the screen. I’ll also replace the fuel filter soon. The road test went well—driving feels smoother, possibly better on highways, and the gasoline smell has significantly reduced. I’ll monitor for any return of codes.

So now I have new issues: P0171 (lean mixture) and P0174 (excessively lean mixture). This suggests the engine is now running too lean. Worse, when I let the truck sit overnight, it stumbles upon startup—something that didn’t happen before! I suspect I may have bent one of the fuel lines feeding into the injector spider assembly, possibly allowing air to enter. I’ll carefully re-bend and reinstall them properly. The only component I changed was the fuel pressure regulator. Grrr!

If the truck stumbles when starting, it may point to a fuel delivery issue—double-check for vacuum leaks.

No, I’m confident there are no vacuum leaks. The only lines from the intake manifold are: two to valve covers, one to the brake booster, and one to the evaporative system (which passes through a solenoid). I’ve sprayed ether on all connections—no idle changes. Last night, I reinstalled the original fuel pressure regulator after cleaning its small filter screen. Now it runs smoothly. I’ll have my dad drive it for a few days to see if any codes return. I think I mixed up the diagnostics—I didn’t realize how critical that screen is! When you remove the regulator and turn it over, you'll see this screen—make sure it's clean. Mine was clogged!

Where did you purchase your replacement fuel pressure regulator?

AutoZone—I know, I know. But it was the only place open at 8:30 PM! I’m returning it today.

Stores stay open late just to handle returns—consider visiting the dealer parts department if cleaning doesn’t resolve the issue.

The clogged fuel pressure regulator screen was likely the root cause—the codes have disappeared, and the truck runs smoothly now. Fuel efficiency has also returned to normal.

I like my pizza with extra cheese and a variety of meats! Great job—happy to hear your truck is back to full operation!