Finally, a test box to check your gauge before going to all the trouble of pulling it! This test box will check fuel gauges, flap gauges, cht and oil temp gauges on Stewart Warner, AC and some Rochester (fuel,cht,oil temp) systems. Each test box will come with full instructions on how to use and any settings you need to know. For example, disconnect your S/W fuel sender wiring and hook your test box to the sender's wiring and ground. Power on the system, and turn the selector switch to the 30 ohm setting, now, if your fuel gauge is working, it should read FULL. Dial up 105 ohms and you should be at HALF tank. Continue and turn the knob to 240 ohm setting and your gauge should read empty! If your gauge is not very accurate, send it to us for calibration. This box will pay for itself many times over the very first time you use it. Troubleshoot your system the easy way, save those skinned knuckles and possibly other damaged components by trying to remove a working gauge. SORRY, PRODUCT AVAILABLE TO US FAA CERTIFIED MECHANICS AND REPAIR STATIONS