Replaced the battery in 05 F150 after replacing the security alarms, horn did not work, then the emission light came on. Pushed the button on the left under the dash and the horn came back but not sure why the emssion light, just passed smog a couple months ago. Any ideas?

 

Going to need to have it scanned to find out what the code is to know what it is.

Also might want to have this moved ( by the moderators ) to the correct engine forum, this is more along the lines of an engine question than an electrical question.

 

how is this more of an engine question??
His electronics are not funtioning correctly. i.e. relays, brain etc.
His engine runs fine from what I can see. He is not complaining of an engine not starting, or it knocking or stalling etc.

 

If you read it, "emission light came on" Which this would be the CEL or Check Engine Light, which one would need the code to know what is going on.
If the OP can get the code read at Autozone, it is going to be a DTC in the Pnnnn format, which are Powertrain diagnostic trouble codes.
Could be a Bnnnn range ( body ) code, but Autozone usually does not have a scanners that can read these types of codes, and these are rare in the forums here, when compared to the number of Pnnnn DTCs.

You are making an assumption that this is a " relays, brain etc" problem, which is a very vague statement to post any time, let alone with this level of detail. In general just conjecture on your part.

The CEL indicator is illuminated, the engine is not fine, there is a problem. An engine problem does not need to stalling, knocking, misfire or not starting to be an engine problem.
Your list of conditions to constitute an engine problem are < 50 of the 1000+ DTCs that the truck can have just in the Pnnnn range.

So while the OP might not be complaining of drive-ability issues, the truck is.