Codes and Cats

Discussion in 'General Chevy & GM Tech Questions' started by Goldie, Jun 12, 2013.

  1. Goldie

    Goldie Epic Member 5+ Years 1000 Posts

    My truck has the 4.3L engine and recently threw two codes (P0171 and P174), lean combustion bank 1 and bank 2. The codes have been cleared and the truck runs well. I've been told that I may need new 02 sensors eventually. So my question is how can I tell if the sensors that may need replacing are before or after the cats?
  2. dsfloyd

    dsfloyd Epic Member 5+ Years ROTM Winner 1000 Posts

    Since those are lean codes also look at the MAF. I know with my aftermarket Granetelli MAF (supposedly designed for cold air intakes) those are the codes that I will throw sometimes if coasting down long hills and it is solely related to the MAF (on mine). Granatelli even said it was an issue with some trucks with intakes and needed a different MAF to correct it. If it is the o2 sensor it would be before the cat. I am not exactly sure how to test the sensor itself. Being you are getting both p0171 and p0174 I would lean towards something before the o2 sensors.

    Here is a description
    A code P0171 may mean that one or more of the following has happened:

    • The MAF (Mass Air Flow) Sensor is dirty or faulty
      Note: The use of "oiled" air filters may cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles[​IMG] where the MAF sensors leak the silicone potting material used to protect the circuitry.
    • There could be a vacuum leak downstream of the MAF sensor
    • Possible cracked vacuum or PCV line/connection
    • Faulty or stuck open PCV valve
    • Failed or faulty oxygen sensor (bank 1, sensor 1)
    • Sticking/plugged or failed fuel injector
    • Low fuel pressure (possible plugged/dirty fuel filter!)
    • Exhaust leak between engine and first oxygen sensor
    Possible Solutions

    A lot of times, cleaning the MAF sensor and finding/fixing vacuum leaks fix the problem. If you're on a tight budget, start there, but that may not be the fix for certain. So, possible solutions include:

    • Clean the MAF sensor. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling
    • Inspect all vacuum and PCV hoses, replace/repair as required
    • Inspect all hoses and connections in the air intake system
    • Inspect and/or test the intake manifold gaskets for leakage
    • Check for a dirty fuel filter and proper fuel pressure
    • Ideally you'll want to monitor short and long term fuel trims using an advanced scan tool
    • If you have access, you may want to run a smoke test
  3. Goldie

    Goldie Epic Member 5+ Years 1000 Posts

    Thanks for all the great information. It gives a lot to work with and has saved me some from "huntin' n' guessin' ". As an FYI I cleaned the MAF with CRC MAF cleaner less than 100 miles ago.
  4. Goldie

    Goldie Epic Member 5+ Years 1000 Posts

    The problem is fixed and I wanted to say I learned a lesson ---> slow down and pay attention to what I'm doing. The problem was I had to pull the air tube of my K&N to get the MAF so I could clean it. The air tube has a rubber hose slip on on the back side of it. This goes to the top of the valve cover. I had to take this hose off and simply forgot to attach it. The truck runs fine now. And again, many thanks to [MENTION=26043]dsfloyd[/MENTION] for the great write up!
  5. dsfloyd

    dsfloyd Epic Member 5+ Years ROTM Winner 1000 Posts

    Glad to help, even though I grabbed it from a website:). I've done is liar things with the MAF. I have left it disconnected once on two different vehicles. On the car it was noticeable immediately. On my truck I didn't notice it until I went to get on the highway. Went to hit the throttle and then it started sputtering. Knew exactly what it was when it happened but things like that happen. Glad it was as simple as reconnecting a hose.

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