When the reading is wrong

Where does it sit?

Run the first reading and hold white paper, then black tape, 3 mm from the domes. A working block gives a short bar over white and a long one over black. If it does not, where the numbers sit says where to look.

Pick what you see. The dark bars are where a working block reads, from this book's model; the red bands are where the fault puts it.

At the top: nothing pulls it down

SIGNAL starts at VCC and only light coming back pulls it down. So a reading stuck at the top means the emitter is dark (GND not connected), the light goes the wrong way (the back of the block faces the surface: the sensor looks out of the front, the side with the pin names), or the surface is too far.

At the bottom: too much light

Near 0 over black tape means the receiver is lit by something that is not your surface. The filter in its dome blocks visible light, not infrared, and the sun and filament bulbs are full of it. The emitter is always on, so the board cannot tell its own light from the room's. Shade it: a short tube of black card, a skirt on the robot, or simply closer to the surface.

Both the same: the ink, or the height

If white and black read alike, the black is pale in infrared, or the sensor is too far or pressed flat against the surface. Try matte black electrical tape at 2 to 5 mm.

Wandering: a floating pin

A reading that drifts or follows your hand is connected to nothing that holds it. Check the wire is on SIGNAL, the right-hand pin, not NC beside it, that the sketch reads the pin you wired, and that VCC is there. Then check VCC is your board's logic voltage: 3V3, or 5V on an Uno.