The Internal Revenue Service recently released at private letter ruling (PLR) regarding auxiliary power units (APUs). An unnamed taxpayer had asked whether the APUs it installed on its tractors were “parts and accessories” subject to the 12% federal excise tax on new trucks. The APU package consisted of: (1) a two-cylinder diesel engine and an air-conditioning compressor; (2) a fuel-fired bunk heater, evaporator and controller that are installed in the cab, sleeper compartment or tractor tool box; and (3) a condenser usually mounted on the back of a cab. In PLR 200711006, the IRS chief counsel's office reasoned that the package added to the utility of the tractor in ways that made it a taxable part or accessory. Although the ruling applies only to that taxpayer, it is likely the IRS would rule similarly with regard to other taxpayers.