Volt and ELR with No Charge Condition Due to Loss of High Voltage Isolation

2011-2014 Chevrolet Volt and 2014 Cadillac ELR models may not charge and have DTC P0AA6 (Hybrid/EV Battery Voltage System Isolation Lost) set in the Hybrid/EV Powertrain Control Module 2 (HPCM2). These conditions may be caused by a loss of Hybrid/EV Battery Pack coolant or a loss of high voltage isolation within the battery. This is one of the most common issues seen on the first generation Voltec system.

 

The Volt and ELR have a high voltage isolation monitor feature where the Battery Energy Control Module (BECM) high voltage isolation monitor circuit tests the resistance between the high voltage positive and negative direct current (DC) bus and chassis ground. The HPCM2 requests the BECM to perform the test.

 

If these conditions are present, the “Service Hybrid Battery Charging System” message may be displayed on the Driver Information Center, the charge status indicator on top of the instrument panel may be amber, and the Check Engine lamp may be illuminated.

 

In addition to DTC P0AA6, the following DTCs may be set:

  • DTC P1FFF (System Isolation / Coolant Level Sensor Fault – Hybrid/EV Battery Charging System Disabled) may be set in the HPCM2
  • DTC P1E00 (Hybrid/EV Powertrain Control Module 2 Requested MIL Illumination) may be set in the ECM.

 

DTC P0AA6 signals a loss of isolation detected within the battery pack itself. This is fundamentally different from the P1AF0, P1AF2, or P1E22 loss of isolation DTCs for the larger high voltage system.

 

DTC P1FFF is an umbrella code that prevents charging for many reasons, but it will always set with another DTC when codes are first read on a vehicle. DTC P1FFF by itself does not point to the issue that needs to be addressed.

 

DTC P1E00 is the HPCM2 request to the ECM to illuminate the Check Engine lamp.

 

Check the Coolant Level

 

Check for any loss of coolant by inspecting the coolant level in the underhood high voltage battery coolant reservoir. The coolant level should be between the top of the label (Fig. 15, #1) and the bottom of the low fill line (Fig. 15, #2).

 

Fig. 15

 

Also remove the drain plug for the battery pack (Fig. 16, #1) to see if any water/coolant drains out, indicating a coolant leak in the battery pack. Refer to PIC5920G for more information.

 

Fig. 16

 

TIP: If coolant is found inside the high voltage battery pack, the entire pack will need to be replaced. This is a very rare condition. Contact the GM Technical Assistance Center if any moisture is found during the drain plug inspection. The battery pack is currently on restriction.

 

Latest Calibrations

 

If a coolant leak is not observed, update the HPCM2 and BECM with the latest calibrations in TIS2Web. The revised calibrations are designed to improve the measurement accuracy of the internal loss of isolation DTC P0AA6. This is the more common repair for this DTC. The HPCM2 and the BECM work in conjunction to monitor and control the battery pack.

 

The BECM programming procedure is unique. It’s important to wait for service programming to fully complete with the checkered flag, as each of the four Battery Interface Control Modules (BICMs) are subsequently flashed once the status bar reaches 100%. Remain patient as this programming could take an additional ten minutes.

 

If DTC P1FFF does not clear after programming, attempt a secured high voltage code clear, as a function of the HPCM2.

 

The special labor operation listed in PIC5920G as well as normal programming updates for the HPCM2 and BECM are covered under the Voltec warranty period (8 years/100,000 miles, U.S., or 160,000 kilometers, Canada).

 

– Thanks to Lane Rezek

 

 

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