Unlike simpler stud finders, the FatMax 400 has a layered interface. Here is the verified legend straight from the manual.
Even with the manual verified, the Stanley FatMax 400 has limits.
Q: Can the Stanley FatMax 400 find studs through two layers of drywall? A: No. The verified max depth is 1.5 inches. Two layers of 5/8" drywall equal 1.25 inches—tight, but possible. Add texture or paint, and you'll need the DeepScan mode. Beyond that, you need a professional scanner.
Q: Why does my unit beep when I lift it off the wall? A: That is normal. The sensor detects the sudden change in density (air vs. wall). The manual verifies this as a "release tone." It is not an error.
Q: Is the Stanley FatMax 400 discontinued? A: The original 400 model has been superseded by the FatMax 500 and 700 in some regions, but the 400 is still widely available and supported. The verified manual remains active on Stanley’s support site as of 2025.
Q: How do I get a replacement manual? A: Go to StanleyTools.com > Support > Manuals. Type "STHT77404" into the search bar. Download the official PDF. That is the only verified digital source. stanley fatmax stud finder 400 manual verified
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Battery Installation: Open the battery door on the back and connect a fresh 9-volt alkaline battery. Calibration:
Place the unit flat against the wall in a vertical position. Unlike simpler stud finders, the FatMax 400 has
Press and hold the activation button. You must keep the unit still until calibration is complete (usually 1–3 seconds).
The unit will beep, and the LCD will display a "READY" icon when it is calibrated to the surface. Scanning:
While continuing to hold the button, slide the device slowly across the wall.
The sequential LEDs and digital display will indicate the edges and center of a stud.
When the center is found, the unit will beep and typically illuminate a center-finding light or arrow. Key Features Q: Can the Stanley FatMax 400 find studs
LCD Digital Display: Shows real-time information such as edge detection, center finding, and battery strength.
Deep Scan Mode: Allows for detection through thicker wall materials.
AC Detection: Alerts you to the presence of live electrical wires behind the wall to prevent accidents during drilling.
Accuracy: Provides wood stud detection accuracy up to 1/8 inch and metal detection up to 1/4 inch. ⚠️ Critical Safety Recall Notice In 2009, certain FatMax Stud Sensor 400 (77-730) Go to product viewer dialog for this item.
units were recalled due to a software error that could lead to improper calibration, potentially failing to detect live AC electrical wires.
How to check: Open the battery cover. If you see a date code starting with "R", your unit is not defective.
Action: If there is no "R" inside, stop using the device and contact Stanley Tools Support at (866) 215-1132 for a free replacement. Troubleshooting Tips How To Use a Stanley Stud Finder