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The latest fad in Prius failures is the dead instrument cluster (combination meter): we have repaired two and since heard of others across the country. Our first hand patients happened to both be cabs (both 2007 models, with 119k and 156k miles, see video below); a little digging on ye ol’ Internet revealed another cab with this problem (2006 model year with 182k mi) via PriusChat. My salvage contact (Steve at AutoBeYours.com) also reported cluster failures from a cab company that buys from him.
A case posted on my beloved iATN (link for iATN members) came from a dealer tech in Cordova, TN, on a 2008 with less than 15,000 miles (effectively brand new) which he eventually attributed to cold temperatures. We have several customers with high mileage, second-generation Prius with no such problems (e.g. our favorite Prius, see blog) and know of lots of similarly trouble-free Prius in much colder places than Tennessee. There’s speculation about coffee (or other liquid popular with cabbies) getting spilled on the dash—the plateau before the instrument display makes a nice shelf for such things—that may be the missing link and a likely secret. For now the root cause remains unknown.
The primary symptom is a dark instrument display, center speedo/combo as well as peripheral indicators (including “ready” light), with the exception of the “check engine” light, turn signal indicators, and security light (explanation of this to follow). The cluster may go down while driving or at initial start-up. Sometimes the condition coincides with a batch of interesting behaviors: the reverse lights, power button, and rear hatch release are inoperative. This is especially disturbing since it makes it challenging to turn the car off.
The solution is replacing the combo meter/cluster. Extricate the top dash panel, flip it over, and remove the white plastic cover to expose the speedo circuit board. New ones are available from the dealer for a mere $711 (special order) and can be programmed with the original mileage. With ninety minutes labor, the total bill is $951. The other option is a used board; while cheaper ($250-ish) they cannot be reprogrammed and will read the mileage of the host vehicle. If this is less than your car, it could be a bonus. If not, not. Total bill with labor, $451.
That said, what’s the explanation for the weird behaviors? It’s mostly confusion over vehicle speed: the speedo acts as a gateway to broadcast related information to other “body” ECUs on a dedicated communication bus called BEAN (Body Electrical Area Network) including those in charge of the power switch and the rear hatch. If the vehicle is moving (or it can’t tell), the Power Source Control ECU interprets brief presses of the power switch as accidental and ignores the request, a safety feature to keep you from inadvertently shutting down the car when moving. If the power button is held down, the car interprets it as an emergency and shuts down no matter what speed. This function still works even when the instrument cluster goes down. 
Likewise, the vehicle will prohibit the hatch from opening if it doesn’t know what speed its going. Reverse lights are not related to speed, simply controlled through the combo meter according to gear inputs from the HV ECU.
The “check engine” light, turn signals, and security lights continue to work because their circuits are independent of the main computer processor inside the assembly (see wiring diagram, pictured).
Video of our second troubled cab, before the Power Switch stopped working:
Video of the same cab when the Power Switch finally acted up:
Video of case posted on PriusChat:
