Power Supply / Noise Problem?
Assuming none of the above checks have already pin-pointed the cause, check the power supply to the DPM. Is the power supply adequate? (You may need to check this with an oscilloscope for a LED meter) Is there a noise problem? This is often characterized by the meter reading the correct value most of the time, but occasionally the reading jumping significantly, before resuming a constant level, or just general instability, or sometimes failing to fully zero, when the inputs are shorted. Try connecting a capacitor (10_F) between V+ and COM. Next, try disconnecting the power supply to the meter and replacing it with a battery fed supply. If instability disappears, this would indicate that noise is being transmitted through your power supply. Ideally we would recommend that a good quality linear power supply is used. If the meter starts to respond correctly, having previously been reading incorrect values, this suggests a wiring issue, in that the inputs to the DPM are referenced to your
