Engine runs constantly

Can anyone assist, My 2012 prius with 103 000 miles runs constantly although the 12v battery was replaced last week, the day after the new battery was installed it run perfectly with the engine shutting down as usual, but ive realised that my MPGs have gone down slighly, Oil was replaced at 100, 000 miles and plugs at 98,000 miles. When brakes is applied and vehicle is brought to a halt you would hear like the engine shutting down but it would immediately restart, its like something is preventing the engine from shutting down yet at times when vehicle is in motion it would go into EV mode, very strange behavior. any suggestions would be very much appreciated?

 

Both the AC and the heater was not running during this event, also traction battery charge level was full, The battery fan nor the EGR has never been cleaned, could this be the issue ?, yesterday the engine light came up and engine refused to shut down even when stopped, read codes were $07E8 and $07EA, scan tool cleared the codes and now engine shuts down after codes were cleared but what does this code mean? Engine shuts down at times but I still realise mpg drop a bit, could this be fail or failing O2 sensor?

 

Dont have any pets which may contribute to clogging of intake, neither do I drive in dusty environments

 

Ok so today engine check light came on the did a scan result P0A7F, The code was cleared and I currently monitoring to see if code returns, If it does I guess I will have to replace one of the battery cells, If anyone have ever experienced this I would welcome your troubleshooting procedures and what was done to solve the issue.

 

The battery level was full and the car moved via electric propulsion, so I guess all 4 quaters, Im wondering if it can be the battery terminals for the bus bar?

 

I dont have a choice I guess, what would you recommend

 

Where I live shipping and customs for an entire battery pack would be cost prohibitive also there is no one on island who specialize in hybrids or electrics so Im left to do everything myself, I should be recieving the ODB2 bluetooth adapter as well as ODB2 - USB cable to assist with troubleshooting, I have already gotten Dr. Prius and downloaded Tech Stream so Im currently awaiting the hardware, It takes a while about 2 weeks after purchase to arrive on island, I have decided to go the route of replacing individual cells if it infact are the cells, but will test first as suggested, I do not have a cell balancer though.

Thank you all for your suggestions, Im reluctant in opening the pack until I test it first but also because my experience in this field is limited, but I have watched countless youtube videos.

 

2012 prius with 105 000 miles

 

I have already gotten two cells just in case, seller mentioned they were off a 24 000 mile donor and shows voltage of 7.9V, if they are higher than whats in the car would I be able to just drain them to the same voltage as those from the pack and replace without balancing with specialized cell balancer, I currently do not have a cell balancer

 

Im from St. Lucia,