Brought my 2004 yukon for new sticker. It failed emission test . It cannot communicate with the obc. I was told to drive it for around 50 mi which I did . It failed again. Then was told there was now power at the connector under the dash. Changed the bat about thee mounths ago,no check engine light at all. Is it possible a fuse could be involved, also checked connector and all wiring is tight.

Check all the fuses first. Let us know if you find a blown one.

Check all the fuses first. Let us know if you find a blown one.

Is there 1 particular fuse that responsible for the obc thanks

Let's get out terms straight. OBDII stands for On Board Diagnostics. That's the universal software that's installed in cars after 1996. The computer in your car is called the Powertrain Control Module (PCM). If you look under the hood in the fuse box you should find a fuse that's labeled PCM or PCMB (for PCM Battery). That's what power the PCM. If the fuse is good, then check the black/white ground wire located in the engine compartment rear on passenger side. Make sure the ground is clean.

You didn't tell us which engine is in this vehicle, so the wiring may be different.

Is ther 1 particular fuse that responsible for the obc thanks

Some models share a fuse between the OBD connector and the cigarette lighter.
Check that nothing is shorting out the cig lighter socket.

Captain rich Gets the gold star It was the cig lighter fuse! Put new fuse and regained power to the connector.

Great job, CaptainRich.