Does anyone know if a heater that's blowing only cold air is due to the upstream sensor not working

 

If the heater is only blowing cold air, it is usually due to a broken heater control valve or cable. But, also can be due to a clogged heater core. See this other thread for the location of the valve (under the hood, near the back). See if it moves when you select heat. You can also manually move the lever in the engine compartment and see if you get heat.

https://www.hondaaccordforum.com/for...81/#post366033

 

Or even possibly a stuck open thermostat.