2008 Prius will not start or go to accessory mode after discharging and recharging 12V battery

Tiny
THE_VARELAS
  • MEMBER
  • 2008 TOYOTA PRIUS
  • 1.5L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 110,000 MILES
Car was left unattended in garage for 2 weeks. Guess we may have left lights on or door open, because the battery appeared to be dischared when we got back. Charged it on a charger supposedly set for 2 A. Battery now works fine if we move it into our other prius, and battery from good prius doesn't do anything in malfunctioning prius. Also had the 12V battery that supposedly discharged tested professionally, and is said to be good. We also recharged the battery using a jump start after 8 hours on the charger. (Note that the charger now does not appear to work, not sure it ever did - and we are wondering if it fried something).
Symptoms:
1. Pressing power does nothing - EXCEPT when there is no key - then it shows the no key icon on the instrument panel, but nothing else. Neither green led nor amber led on start button light.
2. Door open icon works, but no sound when open door with key in slot. On our other working 2008 prius, we get beep beep beep when we open the door with the key in slot.
3. OPeration of "no key" icon is different than with working 2008 prius.
a. Unlike working prius, if the wrong key is in the key slot, I do not get a "no key" icon when I press start. I only get a "no key" icon when there is no key in the slot.
b. If the right key is in the key slot, I do not get a "no key" icon - same as good prus.
c. If the right key is in the interior of the car, but not in the slot, I do NOT get a "no key" icon", which is same as working prius
d. If the wrong key is in the interior of the car, but not in the slot, I DO get a "no key" icon same as working prius.

4. We checked all fuses that seemed relevent - ECU-IG, AM1, IGN, ACC-B, PANEL, ACC, EFI, HEV, P_CON_MAIN, ABS-1. DOME, AMP, ABS1, 2, 3, DC/DS-S. All have negligible resistance.

5. Headlights work, interior lights work, brake lights work, horn works. Correct key locks and unlocks the doors remotely.

6. We measured the voltage across AM1 fuse with fuse removed, and got an oscillating voltage between 0 and 15 V, unlike the working prius which has 12.5 V there. Google said that this is classic symptom of short circuit in the inverter coolant pump, so we tried disconnecting the inverter coolant pump electrical connection, and the oscillation stopped- and we got 12.5V across empty AM1 just like working prius, but still no response when push power button.

7. ODBLink port powers on, but cannot connect to car computer.

8. Not sure if relevant, but when I open the driver side door, to get in initially, I hear two relay closures from the left front of the car - click click, click click. This doesn't happen on the working prius.

Did we fry the Power source ECM? Or is there something else related to the key recognition that is broken? Could this have anything to do with the hybrid battery? It was rated "fair" condition a few years ago - and we did have some red triangles that came up and went away in very cold weather some times, although very infrequently, especially this year.
Monday, September 22nd, 2025 AT 4:42 PM

12 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
So these cars had a common problem and even a recall on the inverter coolant pump shorting out which wont let the car boot up, a classic Gen 2 Prius problem dragging AM1 down. The hybrid battery should be okay. Lets disconnect the coolant pump and try the power button to see what happens. The OBD scanner should then connect. Check out the images (below). Let us know how it goes
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Tuesday, September 23rd, 2025 AT 8:30 AM
Tiny
THE_VARELAS
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Hi Ken. We tried that in #6 in our description of things already performed/tried. As we said we went from an oscillating voltage reading of 0v-15v. To a steady 12.5v across the empty AM1 fuse location. But nothing else changed. Any other suggestions?

And some additional information. Checked the fuse-able links in the fuse box in the engine compartment and did not see anything broken, burnt, etc. If there is a safe way to electrically test those let us know.

One additional comment. The 12V battery might have never been discharged. When the car did not start we automatically assumed it was the 12V battery & hooked it up to a charger that quiet frankly might have never worked to begin with. It's not working now. Only thing we know for sure is that the next day the 12V was tested at AutoZone and tested not only good but with a full charge. And our own testing of the battery with a simple hand held meter shows it at 12.5V. Additionally when that 12v was put into a working 2008 prius & diagnostic done on the MFD it read at 14V. We did also try jumping it with the working prius & believe we attempted that correctly per the well know instructions of using the terminal in the fuse box in the engine compartment.
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Tuesday, September 23rd, 2025 AT 8:52 AM
Tiny
THE_VARELAS
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  • 8 POSTS
I am thinking we check the IG1 relay next. Any other suggestions?
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Tuesday, September 23rd, 2025 AT 2:08 PM
Tiny
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The door locks work, both from the key fob and the button in the door - but I guess that would still be compatible with IG1 relay failing to close - since the door locks work without power switch being pressed and IG1 is activated by power switch. We also saw some advice to disconnect both the 12V battery and the orange connector on the hybrid battery, and let it sit to reset the computers. WE will try that tomorrow.
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Tuesday, September 23rd, 2025 AT 6:04 PM
Tiny
KEN L
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  • 54,137 POSTS
When you scanned the code for codes did you use a CAN scanner? If not that is what we need to do, here is a video.

https://youtu.be/u-4syLc-ifQ

Please let us know what happens.
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Thursday, September 25th, 2025 AT 10:06 AM
Tiny
THE_VARELAS
  • MEMBER
  • 8 POSTS
Given our situation of the car basically being a brick right now, this tool will not need it to actually go into ready/ACC mode or some other mode it can't go into right? Otherwise it defeats the whole purpose for buying it.

Also are there any brands or models you recommend specifically for Gen 2 Priuses with all their uniqueness's? I see scanners cost anywhere from $25 to $200. Are all OBDII code readers by default CAN or do we need to buy one that specifically says it's CAN?
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Thursday, September 25th, 2025 AT 11:04 AM
Tiny
THE_VARELAS
  • MEMBER
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Note: we tried to use our ODBLink XL tool - I think this is the same as what you are referring to as a CAN Scan? We plugged it into the ODB port -- the ODBLink app says it cannot connect to the car - although the various leds on the ODBLink XL tool flash to show communication happening.
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Thursday, September 25th, 2025 AT 6:24 PM
Tiny
KEN L
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Do the cars headlight come on?
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Friday, September 26th, 2025 AT 9:16 AM
Tiny
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  • MEMBER
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Yes.

Below is what should be a concise list of what works & what does not. Thanks.

When the power button is pressed with a working key fob inside the car nothing happens:
No green LED on start button.
No starting of the car.
No going into ACC mode.
No Ready indication.
No display on the dash (Combination meter items like fuel gauge, odometer, gear, speedometer AND no Maint reqd, check engine, TMPS, ABS, key in car, airbag, or Triangle of death flash).

Things that work:
Horn
Headlights
Brake lights
Open door icon
Internal cabin lights
12V battery (verified good)
+++ No key icon (when tried without key inside the car) ***
Door lock/unlock by remote AND by buttons on door

Other Info:
As stated the 12V battery is good.
Both remotes have new batteries and can lock/unluck doors, will trigger "no key" icon when try to start car without them inside the car.
AM2 fuse is good.
Fusible links in the engine compartment fuse box all look good.
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Friday, September 26th, 2025 AT 11:46 AM
Tiny
KEN L
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  • 54,137 POSTS
I would do the CAN scan to see which codes it has, it sounds like the hybrid battery is no good. The scan will tell us for sure,

https://youtu.be/u-4syLc-ifQ

Please go over this guide and get back to us.
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Friday, September 26th, 2025 AT 6:27 PM
Tiny
THE_VARELAS
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The CAN scanner was unable to scan the car.
Why do you think it's a bad hybrid battery causing all the above symptoms and not a bad power control ECU?

The OBDLink LX scanner we used said it was unable to connect to the car. Don't CAN scanners need the car to at least enter ACC mode - which the car is unable to do. If you know a CAN scanner that can read the codes with the car completely dead please provide the brand and model number. Thanks.
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Friday, September 26th, 2025 AT 8:35 PM
Tiny
KEN L
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I have seen these car act this way when the hybrid battery goes bad, but the scanner should work if the 12 volt battery is good, did you have the battery tested at the auto parts store, also are the cables clean and secure? Please go over this guide

https://www.2carpros.com/articles/everything-goes-dead-when-engine-is-cranked

Also, recheck all of the fuses.

https://www.2carpros.com/articles/how-to-check-a-car-fuse

Please go over these guides and get back to us.
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Saturday, September 27th, 2025 AT 10:56 AM

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