High Voltage System Fault, Reduced Propulsion Power
The BOLT-U3620 (HPCM2 (Hybrid Powertrain Control Module 2)) EV fault code means: High Voltage System Fault, Reduced Propulsion Power. This is a serious severity code.
- Keep driving?
- Yes, but fix soon
- DIY difficulty
- advanced
- Estimated cost
- DIY reprogramming via J2534 pass-through and GDS2 subscription: $50-$150 in tool access fees if you already own compatible hardware. Dealer HPCM2 software reflash (sub-symptom 09): $0 if covered under a GM service update or warranty, otherwise $150-$300 in labor. Hardware repair (sub-symptom 17, connector, HVIL wiring): $200-$600 at an independent EV shop. HPCM2 module replacement if reflash fails: $400-$900 parts and labor at a dealer.
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Common Symptoms
- Reduced Propulsion Power message appears on the driver information center
- Acceleration feels noticeably sluggish or limited, especially above 30 mph
- Vehicle enters a limp-home mode, capping available power well below normal
- Orange or red wrench icon illuminates on the instrument cluster
- Check Engine or Service High Voltage System warning light comes on
- MyChevrolet app shows a vehicle health alert or reduced range estimate
- Fault may be intermittent, clearing on its own after a key cycle, then returning under load
Probable Causes (Ranked by Likelihood)
- HPCM2 software over-sensitivity triggering a false fault (sub-symptom byte 09), resolved by HPCM2 reprogramming per GM service updates Very Likely
- High-voltage interlock loop (HVIL) circuit open or intermittent, causing the HPCM2 to detect a hardware fault in the U3620-U362E diagnostic range (sub-symptom byte 17) Likely
- CAN network communication fault between HPCM2 and BECM, causing the high-voltage system to lose confidence in pack state data Likely
- Battery pack connector or high-voltage harness connector partially unseated, introducing resistance or intermittent opens into the HV circuit Possible
- BECM detecting an internal cell imbalance or temperature anomaly and signaling the HPCM2 to derate propulsion power Possible
- Drive unit inverter module reporting an internal fault over CAN that the HPCM2 interprets as a high-voltage system event Less Likely
- Coolant intrusion or corrosion at high-voltage junction box or service disconnect, degrading insulation resistance below threshold Less Likely
Step-by-Step Diagnostic Procedure
Step 1. Read the full DTC with sub-symptom byte using GM Tech 2 / Global Diagnostic System (GDS2), an Autel MaxiSys, or a Foxwell NT530 with GM software. A generic OBD-II reader will not display U3xxx codes or the sub-symptom byte. Record the exact byte value before clearing anything.
Step 2. If sub-symptom byte is 09, check GM Service Updates N232421260 / 23-NA-157, N242437530, and N242443490 against your VIN on the GM NHTSA lookup or at your dealer. These updates contain HPCM2 software reflashes specifically for the false-trigger condition. Reprogramming is the primary fix and requires GDS2 or equivalent J2534 pass-through hardware.
Step 3. If sub-symptom byte is 17, follow GM SI (Service Information) diagnostics for the full U3620-U362E range. This indicates a hardware-level fault. Do not stop at software. Proceed with the remaining steps.
Step 4. Inspect the high-voltage service disconnect (manual service disconnect plug, located in the rear of the vehicle under the cargo floor) for proper seating. Remove and reinsert it firmly. Look for any corrosion, heat discoloration, or melted plastic on the connector body.
Step 5. Visually inspect all accessible high-voltage orange harness connectors at the battery pack, drive unit, and onboard charger for signs of looseness, corrosion, or physical damage. Do not probe or backprobe any orange connector without verified HV safety training and insulated PPE. If you see damage, stop and take the vehicle to a dealer or certified EV shop.
Step 6. Using GDS2, run the BECM and HPCM2 live data stream. Look for any cell voltage imbalance greater than 50 mV between the highest and lowest cell groups, any coolant temperature above 45 degrees C at rest, or any insulation resistance fault codes accompanying U3620.
Step 7. Check for additional DTCs stored in the BECM, TCON, and inverter modules. Codes like P0AA6 (isolation fault), P0D5B, or P1FFF appearing alongside U3620 point toward a hardware root cause rather than a software calibration issue.
Step 8. If sub-symptom 09 is confirmed and the applicable GM service update has already been applied, verify the software part number loaded into the HPCM2 matches the latest calibration in GDS2. Occasionally an incomplete flash leaves a partially updated module that still triggers the false fault. A full module re-flash under GDS2 typically resolves this.
Frequently Asked Questions
What does Bolt EV code U3620 mean?
U3620 is a GM-specific network or hardware fault stored in the HPCM2 that signals a problem with the high-voltage propulsion system. The vehicle responds by limiting available motor power to protect the battery and drivetrain. To know exactly what triggered it you need to read the sub-symptom byte using GDS2 or a compatible scan tool. Sub-symptom 09 is almost always a software calibration issue fixed by a free HPCM2 reprogramming update from GM. Sub-symptom 17 points to an actual hardware problem in the high-voltage circuit that needs further investigation.
Can I still drive my Bolt with a U3620 code?
Yes, but with reduced power. The vehicle will typically enter a limp-home or derate mode that caps acceleration and top-speed capability. It is generally safe enough to drive to a charging station or repair shop under normal traffic conditions, but you should avoid highway merges or situations requiring full power. If the warning is accompanied by a burning smell, visible smoke, or the car refuses to move at all, stop driving and contact roadside assistance.
How much does it cost to fix U3620?
If the fix is an HPCM2 software reflash under a GM service update, the cost can be zero under warranty or goodwill, or roughly $150-$300 in dealer labor if you are out of warranty. If the root cause is a hardware fault like a wiring issue or connector problem, expect $200-$600 at an independent EV shop. A full HPCM2 module replacement runs $400-$900 at a dealership and is rarely needed.
Will my Bolt start and move the next time I need it after this code appears?
Most likely yes. U3620 typically places the vehicle in a reduced-power mode rather than a no-start condition, so the car will power on and move. However, the fault can recur or worsen over time, especially if the root cause is a hardware issue rather than a software miscalibration. Do not count on full performance or range until the code is diagnosed and resolved. If sub-symptom 17 is active and an HVIL or insulation fault is present, drive the vehicle as little as possible until a technician can inspect it.
Sources
This page is built from documented references. Verify against your own service info before repair work.