Most of the time nothing is broken. Work through the causes in the order they actually turn out to be the problem, rather than the order the manual lists them.
These are four different problems with four different causes, and people conflate them constantly.
Check the reference point first, always. If the tool is set to measure from its back edge and you are holding the front against the wall, every reading is long by the length of the tool. This is the single most common cause and it produces a consistent error rather than an obvious fault. Full explanation here.
Then check your angle. Measuring at a tilt returns the hypotenuse, not the distance you wanted, and it always reads long rather than short. That directional clue is useful: if your readings are consistently longer than reality, it is almost certainly angle or reference point rather than a fault.
If the number is wrong in both directions and repeatable, you may have a genuine calibration problem. That has its own guide, and the answer is usually service rather than a home fix.
Change the batteries. This sounds like a brush off and it is not. Weak cells reduce laser pulse power before the tool shows any low battery warning, which shortens usable range and loosens repeatability. Owners regularly describe a tool that was fine last month becoming unreliable, and fresh cells fix a surprising share of those.
Then look at what you are aiming at. Dark, shiny, wet or steeply angled surfaces return too little light for the sensor to resolve precisely. Hold a piece of white card at the far end and measure to that. If the readings tighten up immediately, the tool is fine and the surface was the problem.
Finally, brace the tool rather than holding it freehand. Small hand movements you cannot feel produce exactly this scatter over any real distance.
The tool needs light to come back, and there are four common reasons it does not.
The target is beyond usable range. Not the rated range, the usable one. Outdoors in sun that can be less than half the number on the box. Nothing is wrong.
The surface is absorbing or deflecting the beam. Black asphalt absorbs. Glass, polished tile and water reflect it away at an angle. Measuring to a window typically returns either an error or the distance to whatever is behind it.
There is something in the beam path you have not noticed. A branch, a fence rail, dust in the air on a demolition site, or your own thumb over the aperture. The tool reports the first thing it hits.
The lens is dirty. Fine dust on the emitter or sensor window scatters both the outgoing and returning light. A soft cloth fixes it. Do not use solvent.
Codes are manufacturer specific, so check your manual for the exact meaning, but they cluster into a few families that behave the same across brands.
A signal or reception error means not enough light came back. Work through the target and range causes above. A temperature error means the tool is outside its operating range, which happens in a cold van in winter or a roof in summer, and resolves once it equalises. A tilt or angle error means the inclinometer cannot get a reading, usually because the tool moved during measurement or is being held past the angle it can resolve. A hardware or system code that persists after a battery pull is the one that genuinely means service.
Before treating any code as a fault, pull the batteries for thirty seconds and reinsert them. A surprising number of persistent errors are a stuck state rather than a failure.
Batteries first, and check orientation rather than just charge. Then check the contacts for corrosion, which is common on tools stored in a damp garage or van.
Battery seating deserves special mention on the Leica DISTO D1. The most repeated complaint in its reviews by a clear margin is batteries ejecting from the back and refusing to stay seated. If a D1 keeps dying or will not power on, look at the battery door before assuming the tool has failed.
A display showing segments, going blank in patches, or turning white is different. Owners of the Bosch GLM165-40 report screens failing around the two year mark. That is a hardware fault and no amount of troubleshooting will recover it.
Three behaviours generate complaints and are working as designed, or at least as shipped.
Some units will only store one measurement at a time. Owners of the Bosch GLM100-23 note the model it replaced held several, which is a downgrade rather than a defect.
Some memory functions will not display fractional inches and revert to metric. Bosch GLM165-40 owners flag this specifically, and following the manual does not fix it because it is how the firmware behaves.
And a red dot you cannot see outdoors is not a broken laser. The beam is still working and the tool may still be reading. You just cannot see the dot against sunlight. The physics behind that is worth understanding, because it explains most of what looks like failure in these tools.
If none of the above is quite right, the eight we recommend most often are on our front page.