The Check Engine Light (CEL) WILL come on for a few seconds when you first start your truck while the computer runs it's checks on the systems it monitors. If the CEL comes on and stays on, or comes on while you are driving, then the computer has found a fault in one of the systems it monitors and will store the fault code for 40-75 key cycles, depending on the age and make of the vehicle. You will need to have the fault code checked to see which system has a fault, most major auto supply chains stores (Advance, O'Rileys, Auto Zone, ect) will read the codes for free.

On another note with the CEL, if it lights and stays lit the computer had found a fault in the emmissions system (O2, MAP, ect), if it flashes the fault is in the mechanical system (low oil pressure, high coolant temp, ect). If you get a flashing CEL, stop the car as soon as you safely can, before major engine damage can occur.

 

my light will come on 4 this code p0138 and i kno its and o2 sensor code.the light will stay on 4 about 2 or 3 days but then it will go off and that p0138 code is still reding on my scan tool so what im saying is that the dmv computers will pick that code up and i will not pass insp.

 

The computer senses the malfunctioning O2 sensor every time you start the truck, replace it, clear the code with your scan tool AFTER you replace the sensor, then go for inspection. You will continue getting the code till the defective part is replaced.

 

the light comes on 4 code p0138 wich is and o2 sensor.the light will say on 4 maybe 2 or 3 start ups and cool downs

 

Have you actually replaced the O2 sensor?

 

replace the 02 sensor and it will fix itself