What it means
The hybrid powertrain control module has requested MIL illumination via the inter-module bus. Underlying fault is reported separately by the requesting module.
At a glance
- Usually lights the check-engine light
How it works
The hybrid powertrain control module can ask the engine control module to turn on the malfunction indicator lamp through the vehicle's communication bus. P0AC4 records that request; the cause is a separate fault reported by the requesting hybrid module.
Common causes
Most likely first.
- A fault in the hybrid system has caused the hybrid powertrain control module to request MIL illumination
Symptoms
- Check-engine or hybrid-system warning light may illuminate
- Reduced power or limited hybrid operation
How to diagnose it
- Scan every powertrain and hybrid module, not just the engine control module, and record all codes before clearing anything.
- Identify the hybrid powertrain module's active or stored fault codes and review their freeze-frame data to find the condition that prompted the MIL request.
- Check live data and module status for the specific system named by the companion code, such as battery, motor, or charging operation.
- Inspect connectors or wiring only for the system implicated by the companion code, using its circuit information to guide the check.
- Diagnose and repair the underlying hybrid-system fault, then rescan all modules to confirm the MIL request does not return.
Common fixes
- Repair the underlying hybrid-system fault identified by the requesting module's companion code.
- Restore a poor connector or wiring connection in the affected system.
- Replace a failed component only after the companion-code tests confirm it is faulty.
- Clear P0AC4 after the root fault is repaired and verify that the hybrid module no longer requests MIL illumination.
Can I keep driving?
If the vehicle drives normally and no hybrid warning or power restriction is present, a short trip to arrange diagnosis may be reasonable, but check for companion codes promptly. Avoid driving if power is limited, the vehicle behaves unpredictably, or a separate warning indicates a serious hybrid fault.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.