Is it illegal to operate a commercial vehicle without a horn?

Commercial Vehicles (Trucks anyway) are actually requited to have TWO horns. a road horn (air horn) and a city horn (like a car). Technically, it is illegal to operate the vehicle without both horns working.

Since the horn is part of a commercial driver's daily inspection report, not having a horn would equal a non-working horn. If, on your daily report, you marked the horn as non-functioning, depending on the roadside inspector (and how well his day is progressing) he is within his right to charge the driver with 2 violations. One, for not having a horn in the first place, and a second for making a false statement on his inspection. It's all up to interpretation of the individual inspector.

been driving and owning trucks for 40 years and have never seen anywhere where it states air horn and city horn

law says a horn ,doesnt stipulate what sort just that a horn is working .