Should I replace hybrid battery & keep car, or something else?

Hello Prius Chat - Your take on my situation would be most welcome. How do I know if the hybrid battery is the culprit, when I was getting better than usual mpg, car felt fine until very recently, but all check engine lights on?

This is a 2007 Prius, approx 170K miles. Never had a problem in the 9 years I've owned it... until the last 10 days when all check engine lights and master warning signal light came on 20 miles into an interstate trip. Just had the snows removed earlier that day. Pulled over, turned car off, checked gas cap. When car restarted (easily) master signal and check engine light on, but not (!) or VSC. Car felt like it was driving normally, but I returned home driving slowly.

• Next day: Mechanic said no diagnostic codes showed except possibly a weak 12v battery. He reset the warning lights. No apparent leaks or low fluid levels.
  
  
• 2 days later: 12V Battery check: good .
  
  
• Drove around locally for a few more days, no problems. LIghts stayed off. Car felt normal. Good mpg.
  
  
• Drove to neighboring state, about 80-90 highway miles all check engine lights & master warning signal back on. Stopped, turned off car, checked gas cap. Restarted car. Only check engine & master warning signal on, (!) and VSC went off. Car felt fine and was getting better mpg than usual.
  
  
• On way home from out of state, stopped at Toyota dealership. Diagnostic reader showed no codes. Lights back on after 10-15miles, but otherwise car drove fine except no cruise control. Very good mpg.
  
  
• Driving around town, warning lights go on and off, usually "just" master signal & engine. Sometime also (!) and VSC.
  
  
• Then a few days ago, acceleration started feeling funny during local driving. Car maintained speed while foot off gas, as if on cruise. Also, car accelerated briefly on its own while foot off the pedal. Also, car sounded like engine was racing after driving just 1-2 miles. Fluid levels looked good.
  
  
• Had car towed to dealer for diagnosis.
  
  
• VERDICT: $7700 worth of work needed, including new hybrid battery because 1 cell was failing ($2700). Plus: New brake pads, new front brakes, replace head gasket, fix leaking CV joint, fix rodent damage behind cabin filter, oil leak, inverter something ... among other things. Wasn't able to include link to the mechanic's videos showing everything, unfortunately.
  
  
• What should I do?! Replace hybrid battery with new one? With a refurbished one? Invest in another used Prius for $15K or less?

Many thanks if anyone has any serious ideas about what to do. Money is an object.

 

- Thanks. Yes I have noticed the battery fluctuating a bit just the last few days, but given the car's felt fine and getting good mileage, the hybrid battery dying now does seem surprising ... even though it is original. ... I guess if I sink a good chunk of change into the car I'd keep it a few more years.

 

Thanks for that link and your reply. I am wary of that super long list of repairs. But then again, I've had to do nothing but replace a headlight, the wipers, and the 12v battery, and tires, the whole time I've owned it. The car is still at the dealer and I wouldn't chance driving it home right now. I'll see what more I can find out about the error codes and definitely look into the code reader you suggest. The mechanic was pretty specific about everything else but the error codes. I guess I could ask for some snapshots of their diagnostics, and I'll ask about a voltage sensing harness too!