2012 Prius No READY Mode After Dead 12V Battery – Lost Key, No SMR Click

Hey everyone,

I'm dealing with a frustrating issue on my 2012 Toyota Prius (Gen 3) and could use some advice from the Prius experts here. The car was parked for a while, and the 12V battery completely died (down to 3-4V – wouldn't even unlock the doors or start). I replaced it with a new 12V battery, but now the car won't go into READY mode. The dashboard is a Christmas tree of warning lights, the SMR relays in the back aren't clicking/closing (so no high-voltage engagement), and there's a clear loss of communication between modules. Also, my second smart key seems to have been lost/erased from the system – only the master key works to unlock doors, and I can't reprogram a new one because communication drops during the process.

Here's what I've tried and checked so far:

• Jump-started before battery replacement – no luck.
• Replaced key fob batteries (they were good at 3V, but swapped to new ones anyway) – still only one key recognized.
• Checked all fuses, fusible links, and the hybrid battery – everything tests good.
• Scanned codes after new battery:
  • U0155: Lost Communication with Instrument Panel Cluster (IPC) Control Module
  • U0293: Lost Communication with Hybrid Powertrain Control Module
  • U0073: Control Module Communication Bus 'A' Off
  • U0126: Lost Communication with Steering Angle Sensor
• There was a prior issue with fuel gauge calibration, but that might be unrelated.
• Towed to first dealer: They replaced the brake switch (didn't help), then quoted $3,500 for a new inverter (I declined, as it didn't seem right).
• Second shop diagnosed for 2 weeks: Couldn't pinpoint the issue, replaced the ABS brake cylinder/actuator (no change). They tried to add the second key but lost communication. Also swapped Smart Key Control Module, Power Management Control Module, and Transmission Control Module – still no READY mode.
• Now at a third shop: They're stumped too – can't figure out which module is blocking key programming or preventing READY. Communication keeps failing.

I suspect this stems from the deep voltage drop corrupting something in the ECUs, Smart Key/Smart Box, or immobilizer system. The car powers up to IG mode (dash lights, accessories), but no hybrid activation.

Questions:

1. Any ideas on how to force it into READY mode or bypass the SMR relays for testing? (Safely, of course – I'm not trying to fry anything.)
2. Has anyone seen this exact combo of comm errors after a dead 12V battery? Could it be a fried ECU or immobilizer that needs reflashing?
3. Looking for a skilled Toyota hybrid specialist in the Charlotte, NC / Spartanburg, SC area who knows Techstream inside out, can diagnose ECU/Smart Key/immobilizer issues, and handle reprogramming. Dealer-level experience preferred, as shops so far haven't cracked it.

Thanks in advance for any help or leads – this car's been down for weeks, and I need to get it back on the road!

 

I checked for any signs of vermin damage and everything looks good. All wiring appears intact—no chewing, no green corrosion, nothing unusual. The wires look brand new and fresh.

I also inspected under the dashboard near the ventilation area, and everything there looks clean and untouched.

All the problems started after the 12V battery was drained when the dome/door light was left on. I’m especially concerned about how one of the two keys was erased from the system, but that also happened after the battery issue.

I do have some technical questions I’m hoping you can help clarify:

• Which module/system is responsible for monitoring the 12V battery condition and determining whether the voltage is acceptable for operation?
• The battery measures about 12.3–12.5 V, yet the system still seems to think the 12V battery is not OK. Which module “decides” that the voltage is sufficient and communicates that to the rest of the system?

• At what point, when I press the brake pedal, should the ABS or brake system send or receive the correct voltage to activate the next module and allow the SMR system to turn on?
• Which specific module is responsible for that handoff or authorization?

• How does the 12V power travel from the battery through the system—especially in relation to the brake system and SMR activation?

If possible, I’d really appreciate:
• Wiring diagrams
• A module interaction diagram (showing which module talks to which)
• Any documentation explaining how the 12V system is evaluated and distributed

Lastly, I’m looking for a mechanic or auto electrician in the Charlotte area with Prius experience who can help diagnose and fix this and get the car drivable again. Any recommendations would be greatly appreciated.

Thanks in advance for your help.

 

I’m starting to understand the root cause of my issue:
Deep 12V battery discharge (3–4V) → corrupted EEPROM data → lost smart key registration → security handshake failure → no READY mode + CAN communication codes (U0073 / U0293).

After the severe 12V voltage drop, it appears that EEPROM data in one or more original modules became corrupted, which erased key registration and now prevents system initialization.

Current situation

• Prius will not enter READY mode
• Multiple CAN communication errors (U0073, U0293)
• Only one remaining smart key, but security system is not fully functional

Plan / What I believe is needed

• Restore Toyota security handshake between:
  • Immobilizer (ID Box)
  • Smart Key ECU (Smart Box)
  • ECM
• I have ordered a Smart Key ECU (Smart Box) P/N 89990-47061
• Remaining master key must be properly re-registered to the Smart Key ECU
• Immobilizer seed transfer (EEPROM level, not just Techstream menu)
• Verify ECM VIN and calibration flash are correct and not mismatched
• Possible HV system initialization/SMR relearn/Hybrid battery initialization / safety check

What I’m looking for
Someone who:

• Works with Toyota hybrids / Prius Gen 3
• Understands EEPROM / immobilizer / Smart Key systems
• Can use Techstream + advanced tools (EEPROM work)
• Can diagnose which module is blocking READY mode and fix it properly
  

Location preferred: Charlotte, NC / Spartanburg, SC