My 2008 GMC Yukon Hybrid keeps showing DTC P0AC4 and P0A81. The Hybrid batter pack was replace nearly 2 years ago now and this error only recently started in the last couple of months. I had the battery replaced at Best Hybrid Batteries near me and they have replaced the fan and the computer module in the battery pack under warranty but that has not resolved my issue.

DTC P0AC4 says the Hybrid power control module has requested that the MIL light be activated.

DTC P0A81 says the battery pack cooling fan or circuit has a problem.

I've also see this in regards to the DTC P0A81 code:

Make: GMC
Code: P0A81
Definition: Hybrid Battery Pack Cooling Fan Control Circuit
Description: P0A81 The ignition is ON. Battery voltage is 9-18 V. The fan speed is commanded 35 percent or greater.
Cause: Fan control signal monitor voltage to the drive motor generator battery control module is greater than 2.3 V or less than 0.5 V for 5 seconds. <-- I don't know what that means as a cause.

From what I can tell the fan is working based on what the GaragePro app running on an Android phone. It seems like a nice app for $4 I can see all kinds of information about the hybrid battery from it.

The fan was replaced and is working, the hybrid control module was replaced and is working and the temp sensors all check ok.

I do not see anyone else with this issue from what I can tell by doing my internet searches for these two error codes together.

When the codes are cleared they come back in about a week so something keeps triggering the codes to come back.

If anyone has any clue as to whats going on please let me know.



Current DTC codes

Hybrid function info

 

it says the fan unit has failed

need a new cooler fan for the battery i would not drive it tell it's replaced.


so the PWN controller might have failed (RPM detection part of the circuit ) normally it has 4 pins
12v+,ground, RPM,PWN signal..

 

What is the PWN controller? Is that separate form the battery control module?

Unfortunately I have been driving this for a long period of time in this condition. Luckily most of my driving is short distance lately maybe 20-30 miles is as far as I go.

 

you checked to see if the fan is running correct?
PWN is just a fan controller (it generates a negative voltage i guess to change the fan speeds)which is typically mounted to the fan it self
then you got a RPM sensor (this is reporting 0 RPM to the controller and it's reporting the fan as nonworking)
inside the battery housing if it's still under warranty i'd let them deal with it tell them you have smog coming an it will fail with the check engine light

 

They tested the fan and said it was working. According to the app on the Android phone they fan is showing RPM movement that varies. When I first started my truck today it said Zero RPM but as I drove the truck and it warmed up the fan RPM's went up. SO it seems to me the fan is working however I get a check engine light because the hybrid battery controller says it's not working.

I would think if I can see the RPM's on the app the hybrid battery controller would see the same thing. I'm not sure how I could be seeing the RPM's but it can't. If there was a bad connection I would think I would see Zero RPM's at all times.

 

Interesting that the diagnostic chart mentions the 12V system. So have you had the 12V under the hood checked out? Keeping in mind that the 12V is not used for starting so if it is weak you might not even notice until it is completely dead and then you have a no start condition because it does not power up the hybrid modules. Recently replaced my 12V with a 760 CCA AGM it had completely died but I never saw the codes that you mention. I did get a brake pedal position sensor code so it might be unrelated. Brakes ABS system chit the bed after my 12V battery replacement but that is another sad long story LOL. Check that 12V just to be sure it is not the issue before you deep dive into the Hybrid battery system.

 

I have an AGM 12V battery that is about a year old. It’s an optima yellow top if I remember correctly. I replaced it with an AGM after reading the posts on here about the 12v batteries. I can check it and see what I find out. I replaced it when I was having starting problems in the cold. A parasitic draw was dropping my 12v below 11.5 volts and the truck wouldn’t start. The AGM seems to have fixed that. Other than this problem I occasionally see my dashboard reset when the temperature drops below 60. I believe that is controlled by the BCM but I’m not sure. The battery replacement did not fix that dashboard rest issue.

 

make sure the battery cables are correctly connected

also check the relay under the hood the fuse box closest to the engine.. it might have backed out make sure the fuses/relay are secure.. i had one back out on me like 50% did not cause issue but it was still backing out (little bit of glue fixed that)

 

that is normal

 

Hopefully the 12V is good but if you still have a un-repaired parasitic draw you may have killed it already. If the 12V is good then I would probably move towards having Best Hybrid Battery to check their work if they would agree to it.
Do you think that they actually replaced the fan and module with Brand NEW units? Highly doubtful since their entire business is based upon recycling old hybrid battery parts............. Maybe the USED fan or module that they stuck in there was another bad one or was damaged????