P0AC4 code meaning
Hybrid/EV Powertrain Control Module Requested MIL Illumination
What P0AC4 means
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.
P0AC4 at a glance
- Check engine lightUsually not
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium
- Typical labor0.5 to 1.5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P0AC4
- Check engine light on (caused by the underlying hybrid fault)
- Further codes present only in the hybrid controller
Common causes of P0AC4
- Most likelyEmission-related fault stored in the hybrid controller
- Less likelyRequest still present because codes were cleared only in the engine controller
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Hybrid powertrain cmModule communication bus
What each part of P0AC4 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| A | Area: hybrid propulsion. |
| C4 | The exact fault in that area. |
What to do next
- Note P0AC4 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0AC4
What does P0AC4 mean?
P0AC4 means "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. It is a generic code, so it means the same on every make.
Is it safe to drive with P0AC4?
P0AC4 does not normally turn on the check engine light by itself. Watch for the symptoms above and have it checked soon.
Will P0AC4 fail an emissions test?
Yes, it can. P0AC4 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P0AC4 to fix?
Difficulty: medium. A typical repair takes 0.5 to 1.5 hours of work. This is a job for a workshop.
Will P0AC4 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.