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2003 GMC Suburban 5.3L Flex Fuel: P0171 & P0174 Codes - Diagnosis and Fix

Model: 2003 Suburban Fault Code: P0171 Posted: 2006-12-17 11:54

I recently purchased gasoline at a 'new to me' station overnight, and the next morning my check engine light came on. The OBD2 scan revealed codes P0171 (Rich Mix Bank 1) and P0174 (Rich Mix Bank 2). I drained the fuel tank and refilled it at my regular station—after one day, the light turned off only to reappear the same day with the same codes. I replaced both the air filter and fuel filter, and cleaned the Mass Air Sensor (MAS) using brake cleaner. After these steps, I cleared the codes. The next morning, while warming up in the driveway, the check engine light came back on. I performed a propane test to check for intake or vacuum leaks—no noticeable RPM changes were observed. I added a bottle of Iso-Heat fuel additive and cleared the codes again. However, the light returned the following day during highway driving. The vehicle has 77,000 miles on it. I'm seeking advice on how to properly diagnose and resolve these persistent P0171/P0174 errors in a 2003 GMC Suburban with a 5.3L flex-fuel engine. Any insights into potential causes or solutions would be greatly appreciated. Thanks, Ron

Related fault codes
P0171P0174
Comments (9)
Anonymous 2006-12-18 12:16

Check your fuel pressure—it could be a contributing factor to the P0171 and P0174 codes in a flex-fuel system.

Anonymous 2006-12-18 18:25

Thanks for the tip! Do you know what the proper psi should be for a 5.3L flex fuel engine? Any specific range?

Anonymous 2006-12-19 03:21

The recommended fuel pressure for most GM 5.3L engines is typically between 48 and 54 PSI. However, this may vary slightly with flex-fuel configurations—verify the correct specification using official sources like a dealer parts department or alldata.com.

Anonymous 2006-12-27 10:57

I’ve been having trouble finding a fuel pressure tester that’s compatible with flex-fuel vehicles. Several models I've tried list warnings against use on flex-fuel engines—any recommendations for a reliable, safe tool?

Anonymous 2006-12-29 17:10

It's unlikely the gauge itself would differ significantly between fuel types; however, the system specifications might vary in flex-fuel setups. Still, the pressure range should be consistent with standard GM 5.3L engines.

Anonymous 2007-01-01 13:47

With the key on, my fuel pressure reads 48 PSI. At idle, it drops to 42–45 PSI. Is this within a safe range? I don’t experience starting issues—only a rough idle.

Anonymous 2007-01-02 17:29

That seems low for a 5.3L engine. While standard GM trucks typically operate around 60–66 PSI, flex-fuel systems may have different requirements. I recommend confirming the correct fuel pressure specification through official GM documentation or trusted automotive reference sites like alldata.com.

Anonymous 2007-01-06 12:10

The actual issue was faulty intake gaskets—a common problem on 5.3L flex-fuel engines. The repair turned out to be simpler than expected—only took a few hours. The most challenging part was disconnecting the electrical connectors from the fuel injectors. Also, always ensure you're using foot-pounds (ft/lbs), not inch-pounds (in/lbs), when torquing down bolts.

Anonymous 2007-01-08 12:43

So there was indeed a vacuum leak after all—likely from the intake gasket area, which explains both the P0171 and P0174 codes.