P0AC4: Hybrid/EV Powertrain Control Module Requested MIL Illumination

In many hybrids the hybrid powertrain controller has its own emission-related diagnostics but does not drive the malfunction indicator lamp itself. When it detects such a fault it sends a MIL request to the engine controller, which then stores P0AC4 as a pointer. GM documents this in its 2011, 2013, 2016 and 2019 hybrid tables: the engine controller watches the request message continuously once 3 seconds have passed after power-up and logs P0AC4 after one trip, listed as Type A but marked No MIL because the lamp is owed to the original fault. The real cause is therefore stored in the hybrid controller and must be read there. P0A7B is the equivalent request from the battery energy control module.

High severity. Can I keep driving?If the car still runs, drive as little as possible and go to a specialized repair shop. Do not touch the orange cables: they carry high voltage.

Symptoms

Check engine light on (caused by the underlying hybrid fault)
Further codes present only in the hybrid controller

Most likely causes

Emission-related fault stored in the hybrid controller
Request still present because codes were cleared only in the engine controller

What to check, in order

Start with the simplest. You can tick off each step.

Read the codes with a hybrid-compatible scanner and write down the freeze frame data
Make sure the battery cooling air intake is clean
Check the 12 V auxiliary battery, which can also trigger warnings
Leave any high-voltage measurements to a specialized repair shop

Good to know

Repair difficulty: medium
It affects emissions.

What each character of P0AC4 means

P: Powertrain: engine and transmission
0: Generic code: same on every make
A: Hybrid or air and fuel
C4: Specific fault in that system

Frequently asked questions

Can I keep driving with code P0AC4?

If the car still runs, drive as little as possible and go to a specialized repair shop. Do not touch the orange cables: they carry high voltage.

What causes code P0AC4?

The most common causes are: emission-related fault stored in the hybrid controller; request still present because codes were cleared only in the engine controller.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.