A lethargic dog who is sneezing mucus sounds like a good reason to call your vets office and schedule an appointment. Dogs can contract colds and flus, just like us.
Dogs can get colds or a variety sickness that could cause yellow mucus to form. Take the dog to the vet to be diagnosed, so you can start proper treatment.
Sneezing could be a symptom of several problems. Allergies, illness, or a foreign object (such as foxtail)in the nasal cavity. In any case, get the dog to your vet for medication or removal of object before it worsens.
