Current measurement -- OLED module

I've never used a DMM to measure current until yesterday. I made several measurements and all of them were were consistent with datasheet values or basic Ohm's law calculations.

Then I checked the current drawn by a 128x64 pixel monochrome OLED/SSD1306 module. While I don't have datasheet values for the display I'm pretty sure that they draw something on the order of 10mA, depending on how many pixels are lit.

But my DMM read 5.9uA no matter how many pixels were illuminated. It would be nice if this were true but I don't believe it is. So why would the DMM report a value 1000 times lower than expected?

It turns out that on this particular OLED shield the pins marked VCC and GND are not actually connected to anything on the board. They are just there for good looks, I guess. Most of the current appears to be flowing through the reset pin.

Chinese ebay boards... cheap for a reason.

Probably they are connected just fine. But most chips have clamping diodes in the data (or reset) lines. And if a data line has a higher voltage than vcc the display uses that to draw current from. And because you measure you have a voltage drop across the DMM. What range on the DMM do you use?

A trick would be to write to the screen to lit all pixels and then disconnect the data lines (but keep vcc with the DMM in series connected).

You're absolutely right. While the board runs without either VCC or GND connected, the measured current through VCC goes up when I remove all the other connections except for those two (and reset). With all the pixels illuminated it reads 9.7 mA.

I tried both mA and uA settings on the DMM. It is an inexpensive model.

In what other relatively common situations is it difficult to obtain a straightforward current measurement?