Out of ideas

[hungwell3764]

On my 2000 GMC Sierra w/5.3L 4x4 the Service Engine Soon light keeps coming on after resetting with scan tool.Truck runs good with plenty of power and good fuel economy idles smooth. I've changed Mass Airflow Sensor,all 4 oxy.sensors,air filter, tune-up,gas cap,flushed fuel system,and checked all fluids but still comes on after driving a few days any ideas on what else it could be.

[Jsdirt]

A code would help us immensely ......

[txab]

Without a code you're definitely blindly throwing parts at it.

[gmtech4]

holy crap...

[TooTall1]

Why are you resetting it without reading the code(s)?

[hungwell3764]

Codes were read and said o2 sensors were bad and mass airflow sensor bad not much else left on the emission side to replace

[Duelin]

Are you using a code reader or a scan tool? I have never seen a scan tool tell you that an o2 sensor is bad. Or a mass air flow sensor is bad. A scan tool informs you what part of the drive train management system that is being affected by the problem. An example would be an over rich system or a lean system that the o2 sensors can not adjust the parameter enough to satisfy the ECM. So now you have an o2 sensor code set but there isn't anything wrong with your o2 senors. So if you have a scan tool you would look at the o2 sensor values. Is it a rich condition or a lean condition. Or do you have a o2 sensor voltage high or voltage low or voltage out of range? And then you have to trouble shoot the problem according to the values you get. Another thing to keep in mind. Some of those o2 sensor out there aren't even worth buying.

[Ramin258]

Did you notice when the light comes back on? Right on start up? After you apply a more than normal amount of throttle?

[GMCDriver1]

Yup as previously stated, the scan tool can only tell you what sensor is reading something wrong. Unless the vehicle is super old with a lot of use on it, it is very unlikely the sensor itself is bad, but rather something else upstream that is wrong and thus is reflected in the sensor downstream. You may just want to take it into a GOOD RELIABLE shop, have it diagnosed properly. Throwing parts at it is not the best answer. I've been guilty of that too many times, finally learned my lesson. Also, try to keep in mind that sometimes it can be more than one issue causing the readings so multiple items having to be fixed can happen. Good luck!