06 Prius ~ Red Car with Exclamation Mark

06 with 144k miles

Issue began with ! light & not being able to accelerate past 35 mph with the pedal to the floor.
1. Checked inverter coolant reservoir and the coolant was moving.
2. Checked Air Filter for cleanliness and found a paper towel under the air filter from when car was detailed, assumed was issue. Lights went away and car ran fine for 20+ miles, then lights returned. Noticed rear fan was blowing hard.
3. Towed car to local parts store and had codes checked & cleared codes.
4. Started car codes returned immediately.
5. Cleaned Mass Air Flow (MAF), thinking maybe a paper towel may have caused an issue
Car sounded better, but issue still exist.
6. Tested each cell on the HV battery, 27 cells read 7.9 but one read 6.6
7. Replaced & verified all was balanced.

All was good, for 40-50 miles.
! Light returned, but no symptoms. Except can occasionally hear the rear fan, but not constant as before.

Had codes checked, said 9th cell on the battery was the issue.
Checked every cell. All 28 cells are reading 8V.

12V battery is reading 12.53V.

Does anyone have an advice?

 

Tonight I went through the steps
on PriusDiy to check the 12V aux battery (sorry it won't let me post the link)

And all was good, except when Test 2 was performed I turned on everything I could (air, lights, etc) & the battery dropped to 10.5 for just a second.

Scratching my head cause I can't find a cell that reads low.

Debating on ordering a charger for the hybrid battery & charging the entire thing.

(The car is a single mom & she can't spring to replace the entire battery).

We own 2 Prius ourself so I am sure the charger would be used again.

 

The new cell went in Sunday AM & still reads the same.

Swapped out the 12V battery & within an hour of testing & trouble shooting issue returned.

However, I realized a new symptom, the cruise control stops working when the error code generates & the fan kicks on.
Originally I thought maybe her cruise control didn't work, but it started working after swapping the 12V. Now cruise control is back to not working.

I have an appointment to take it to Toyota for a full diagnostic tomorrow.

 

Diagnostic shows the 9th block is the issue. If I had it to do again would have replaced the entire block not just a single cell.

 

Just an update for those considering using "refurbished" or other hybrid batteries. Spoke with original owner, they had the battery replaced earlier this year by company out of Texas that sends out "service techs". The issue persisted March the battery was replaced again by the company again. Shortly after the car was traded in & that is how my friend acquired the car. The issue started almost immediately when she acquired the car. The company name was not on the battery in the car, however there was sharpie marker where you could tell the battery was originally tested & such. It is a shame that 2 batteries from from the same company went bad in this car within a few months.

We went with a battery from Toyota. Originally they quoted a price of $3357 installed. I asked if they could do any better & referenced them to the price of a local company that offers OEM batteries. My Toyota dealership then offered the battery purchased & installed for $2517.84. Pretty nice they were able to come down & give me a better deal.