Combination Meter Out After 12v Battery Replacement

No. ... Yes.

The CM problem is caused by a failing or failed capacitor. When failing, this capacitor will not function (or fails more reliably!) with the higher voltage supplied by the new 12V battery. Replacing the capacitor, preferably with a higher voltage-rated capacitor (the stock is 16V) and higher capacitance value 220µF (the stock is 110µF) is the fix.

It is madness to destroy your new 12V battery by lowering the voltage to a damaging state of discharge, even temporarily. Just get the CM repaired ASAP.

 

I just had the dark panel thing happen for the first time on my 2007 Prius. The car would not turn off but eventually I was able to reset it by holding down the start button for 5-10 seconds. Worked fine for after that. If I'm understanding correctly, I could also have reset by placing a load on the 12V battery with some odd combinations of lights, brakes etc. but that would be rough on the battery? Did the start button reset also tax the battery? Does that work as consistently as the odd combinations of procedures I've seen detailed here & on youtube? Just wondering for planning purposes because it sounds like this may happen again before I get the CM replaced.. Thx for all the insight so far!

 

This is not a "reset" per se and does not tax the battery. It is a designed emergency shutdown procedure that comes into effect when you hold the power button down for 6 or more seconds. When invoked it initiates a forced shutdown of the system.

 

Welcome. The undersized capacitor in the combo meter is a common issue for the Gen 2. As @dolj explained very well, holding the power button is not a reset. The only reliable remedy is to replace the capacitor. Or put in a rebuilt one from someone like Prius Speedometer Replacement who is a well-respected member here on PC.