My recently bought 2004 Chevy Suburban 1500 is throwing P0171 and P0174 codes. Which indicates lean conditions on both banks. I found the MAF had a busted housing, which let in more air than it should. This has been fixed with a new MAF. I also inspected the air intake components up to the throttle body and cleaned the throttle body. I cleared the codes but they returned about a 100 or so miles later. I've read that the engine may take some time to figure out the condition is fixed. Is this true? Or do I still have a true issue going on?

1 Answer 1


If the codes were truly cleared, you still have an issue.

The fact that they took 100 miles to reappear is a good indication that the codes were cleared correctly, and you still have some sort of lean issue - perhaps so-called "false air" or un-metered air entering the intake.

In my day this was simply called a *vacuum leak". Spraying carb cleaner or venting small amounts of raw propane around all intake and vacuum lines may help isolate the problem. A sudden rise in idle RPM will help locate the leaking component.

While there are a myriad of other causes for P0171/P0174, this seems the most likely given your info and the age of the vehicle. Also inspect the large hoses connected to the MAF, especially the flexible "accordian" sections, as these can have hidden splits and cracks. Which open intermittently due to engine torque movement.

To answer your question, it takes a certain number of specific condition "drive cycles" to complete the so-called "monitor" tests. However, no codes will reappear during this process, unless the condition is still present.

  • Thanks for that. One follow up question. Another symptom I have, is that the engine idles roughly after it is warmed up. Is this related to being lean? Research tells me the fuel trim is off when the computer is trying to adjust it after it is running a while. Seems they could be related to a layman.
    – HunterX3
    Commented Jan 11, 2017 at 12:48
  • Something is grossly wrong. When cold, the engine is run in "open loop" which is a set program of fuel parameters that ignores feedback data from the O2 sensors (that ultimately report your lean P0171/0174 conditions). At operating temperature, the system switches to "closed loop" which alters fuel injection rates depending on feedback data from the completeness of combustion. If additional air is present from leaks that the MAF has not measured, insufficient fuel will be injected, quite possibly causing rough idle, lean running, possibly even ignition misfire.
    – SteveRacer
    Commented Jan 11, 2017 at 13:15
  • Fuel trim, at least "short term", is relevant after the engine (and really O2 sensors) warm up. If the amount of compensation the short term fuel trim can provide is still insufficient to compensate for a large intake leak, the system throws in the towel and sets P0171/P0174. Because you have both codes, it probably is not an intake manifold gasket or specific to one bank -- check further up from the MAF forward. Whatever cracked your original MAF could easily ruin an intake hose as well...
    – SteveRacer
    Commented Jan 11, 2017 at 13:16
  • Only thing in front of MAF is the air filter (new) and air box? Or do you mean further towards the engine? The only thing from there is the large flexible accordian hose that goes from MAF to a large plastic air container (not sure on the name) that is connected to the TB via another large rubber hose. I took it apart last night and inspected everything, unless the hose leaks are so small I can't see them, or maybe the connections? Spraying them with carb cleaner should tell me I guess. I've never tried that trick.
    – HunterX3
    Commented Jan 11, 2017 at 16:22
  • This isn't my ride in particular, but the engine pic shows what my "air inake" system is with the large plastic wing like device. [link]i.ytimg.com/vi/H3orMeHh49s/hqdefault.jpg
    – HunterX3
    Commented Jan 11, 2017 at 16:25

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