Code U1123, Hybrid Battery Problems?

Tiny
MORGIECORP
  • MEMBER
  • 2009 CHRYSLER ASPEN
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 114,000 MILES
I bought the vehicle listed above Hybrid to replace a previous one I lost in a collision. I bought it "as is" from a dealer that said it had run fine until it sat a long time and developed a "hybrid battery fault" and he could not sell it. I ran codes and got a U1123. I bought a new 12V battery and installed it. When I tried to start it, the engine would jump just a bit each time but would not start. I bought a 430V converter and attempted to charge the hybrid battery for over 18 hours. I attached the leads to the two posts under the driver's side passenger seat. It only charged up to 114V but quickly dropped after I removed the converter. When I tried to start it, the engine jumped with more force, but it did not start. I had a new hybrid battery installed by Hybrid To Go but it still won't start and the navigation radio display says, "charge insufficient to start vehicle, leave key in ignition and connect jumper cables". This message is referring to the Charge Assist Mode for the hybrid battery. We tried to charge it according to the manual's instructions but still have the message. The owner of the Hybrid battery company insists the battery is new with Toyota cells inside. Any thoughts?
Tuesday, June 11th, 2024 AT 3:55 PM

17 Replies

Tiny
KEN L
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Does the new hybrid battery lose its charge quickly like the old one? Also be careful charging/checking the hybrid battery, it is high voltage, I would park the car outside until the problem is fixed as well.

The U1123 code indicates a loss of communication with the Hybrid Control Processor (HCP).

Anytime you replace the hybrid battery you need to do a "High Voltage Power Up" procedure. Here is how to change the hybrid battery out correctly and how to do the High Voltage Power Up" procedure and how to do a high voltage isolation test. Wear the appropriate safety attire to avoid getting shocked, thick rubber gloves etc. Also, if you have not already taken a safety course for hybrid battery handling. Check out the images (below). Please upload pictures or videos in your response to any problems so we can see what to help you with.
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Wednesday, June 12th, 2024 AT 12:18 PM
Tiny
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Thanks Ken. The car is at my brother’s place where the hybrid was installed. I’ll check with him regarding the charge on the new hybrid battery. And I’ll pass this information on to him and see if we get any results. I don’t know if the installer followed the steps, you mentioned above but I’ll find out.
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Wednesday, June 12th, 2024 AT 12:48 PM
Tiny
KEN L
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Sounds good, please let me know, I am interested to see what happens.
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Wednesday, June 12th, 2024 AT 1:54 PM
Tiny
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So, we tested the hybrid battery with a voltmeter at the posts under the seat. With the main cables that come from the transmission disconnected and the safety switches connected we only got a.03V reading. The manufacturer said maybe something isn’t connected inside the battery. He is positive the 40 cells are new and charged from Toyota. When read the information on “power up” and isolation codes but don’t understand what exactly it accomplishes. I have the same information on my E-manual for this car.
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Saturday, June 15th, 2024 AT 5:04 PM
Tiny
KEN L
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If the post are reading.03 volts at the main power lugs, then something is wrong with the battery or the service switch is engaged, here are the instructions for the switch service mode. Check out the images (below). Please let us know what happens.
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Sunday, June 16th, 2024 AT 10:49 AM
Tiny
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A new hybrid battery was installed in the Aspen. They used the original battery shell they took out when they put the first hybrid battery in. The car still won’t start.
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Sunday, June 30th, 2024 AT 11:43 PM
Tiny
KEN L
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Okay, did you check the hybrid battery voltage again?
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Monday, July 1st, 2024 AT 10:26 AM
Tiny
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The company guarantees the battery is fully charged but I’m going to check the voltage across the two posts to confirm. If it doesn’t show any voltage what is the chance something inside the battery is stopping the flow of current? For example, the BPCM or connectors as described in the manual. I don’t know if the HV battery has to be connected to the car to be able to read voltage correctly or if it’s isolated (completely disconnected) if voltage can be read. With the original HV battery I was able to read 114V across the two posts at the HV battery. But I think the “smart” cable was connected and the key was off.
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Monday, July 1st, 2024 AT 11:11 AM
Tiny
KEN L
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I would think if this battery does not have voltage in the same environment the old battery did the battery is defective. Here is how to do a battery isolation test. Check out the images (below). Please let us know what happens.
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Monday, July 1st, 2024 AT 11:41 AM
Tiny
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I’ve looked at this battery isolation test and don’t really understand what it does. Can you explain that to me please?
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Monday, July 1st, 2024 AT 11:52 AM
Tiny
KEN L
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I found this video to check out, but it sounds like the battery is no good, let me know how many volts you get.

https://youtu.be/apFEk8xsYas?si=qqxV4cGJ51KOHUF8

Please go over this guide and get back to us.
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Monday, July 1st, 2024 AT 12:00 PM
Tiny
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Hi Ken,
I was able to get the Aspen running and drove it back to Northern Nevada from Gilbert, AZ in one day. However, when I parked it for a day or two it wouldn't start. I kept getting the "System Off" message under the Hybrid display on my radio panel. I found one of the cells/modules in the hybrid battery was only at 5.5 volts while the other 39 were at 7.83 volts. The company that sold/installed the hybrid battery sent me a replacement cell and now my battery reads 314 volts. However, the car still won't start, and the same message appears on the radio panel. Maybe I need to do the "power up" procedure? I think that must reset the HCP. The only code I got is U1123 which I had before I left Gilbert, AZ.
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Friday, June 13th, 2025 AT 4:31 PM
Tiny
KEN L
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Yep, I would try a hard system reboot.

Here is a guide to help:

https://www.2carpros.com/articles/step-by-step-guide-performing-a-hard-system-reboot-for-your-vehicles-electronics-systems

Please go over this guide and get back to us.
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Saturday, June 14th, 2025 AT 8:53 AM
Tiny
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The system reboot didn't sove the problem. However, I found the fuse box (next to the brake pedal on the left side), also called a junction box or relay box, was very corroded. The circuit board inside was full of green junk all over the copper connecting lines and pins. I pulled another fuse box out of a 2008 Aspen (non-hybrid) and installed it. Two different Mopar sites said it was compatible with my 2009 Aspen Hybrid. Also, after looking at the 2008 and my 2009 manual's contacts diagram they appear to be the same. But when I put it in and tried to start it the electric fan constantly ran and I lost communication with the OBDII tool. And it didn't start. Also, the "Hybrid" option under the menu tab on the radio display is gone. I'm trying a reboot again to see if maybe that will reset everything. Any thoughts?
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Wednesday, August 20th, 2025 AT 2:46 PM
Tiny
KEN L
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It sounds like you found the problem but you cant replace the IPM from a non hybrid car into yours. You MUST use the hybrid IPM.
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Wednesday, August 20th, 2025 AT 5:24 PM
Tiny
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What does IPM mean?
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Wednesday, August 20th, 2025 AT 11:44 PM
Tiny
KEN L
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Sorry, the IPM is the fuse panel under the hood. You replaced the junction box inside the car which looks to be the same, I would guess the electrical connectors still have corrosion on them, I would use an electrical cleaner solution to help clean the contacts, or you have a bad replacement part.
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Thursday, August 21st, 2025 AT 7:13 AM

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