Q: Prius master warning light on

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My grandfather was driving a 2006 Prius for a couple of years until the main hybrid battery wasnt good anymore. My uncle paid some guys to install a new hybrid battery and they did the job but 6 months later the battery was dying again and the master warning light was showing. They paid another guy to replace it and 2 weeks later the light was on again.

Fast forward to last week. I took apart the car with my uncle, and we noticed one cell was bad. We purchased, and replaces the bad cell along with cleaning all the copper connectors. Installed everything back into the car on Thursday 9/14. I drove the car the entirety of Friday to ensure nothing was going wrong. The car was driving fine and it felt good. I went to get some groceries and the car's master warning light popped on again along with the check engine light.

If the hybrid battery is now in prime condition, and all cells are at the correct voltage then why is the master warning light still on?

To resolve this most efficiently you have to start with a Toyota re-manufactured, dealer supplied, OEM battery pack. That is the only way to rule out the "possibility" of a fault, of some sort, in the battery pack. Once you have known good OEM power source, it becomes a simple matter of following the flow chart (diagnostic decision tree) in the Factory Service Manual. But, that is a really involved process and so from where I sit there is zero probability of guessing, correctly, as to the exact cause of what you are describing. But, there is a 100% guarantee that if you follow that flowchart, the problem will be found. In your circumstance, you have the complexity of the check engine light. I would suggest resolving the fault behind any OBD-II (check engine light) code(s) that exist prior to resolving the Master Warning light issue. As far as YourMechanic services are concerned, a professional mechanic will resolve this for you, that is get he underlying fault pinpointed, per the Manual, if you request a warning light diagnostic. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.

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