P0AC4 Hybrid control module asked the engine computer to turn on the check engine light

Hybrid/EV powertrain control module requested MIL illumination

P0AC4 in short

P0AC4 means the hybrid control module asked the engine computer to turn on the check engine light. It does not name the fault, so other stored codes matter more. GM bulletins link it to engine-unavailable messages and stalls on the Volt and ELR. Have a technician read every module.

The hybrid control module watches the electric drive and battery. When it finds a fault that affects emissions or drivability, it sends a message asking the engine computer to light the check engine light. The engine computer then stores P0AC4.

So P0AC4 is a pointer, not a diagnosis. Kelley Blue Book says other codes are needed to find the problem. On GM hybrids it often comes with P0AB9 (hybrid system performance), P2797 or P3260. GM tells dealers not to clear codes and to read every module and save freeze-frame data first.

Because the real fault could be in the high-voltage system, the diagnosis belongs to a technician. Nothing on this page asks you to open high-voltage parts.

Most likely causes

  1. A separate fault stored in the hybrid control module

    Common general cause
    Why it ranks here
    Kelley Blue Book says the code only reports that the hybrid control module found a problem; GM bulletins show it set together with P0AB9 or P2797.
    What would change this
    Rises when other hybrid codes are stored. Falls if P0AC4 is the only code in every module.

  2. Engine shut down because of very low fuel or a key-off while driving (Volt, ELR)

    Supported by vehicle data
    Why it ranks here
    GM PIP5008A and PIP5202 tell technicians to confirm the car was not run very low on fuel and that the driver did not switch it off while driving before going further.
    What would change this
    Rises if the fuel ran very low or the power button was pressed while driving. Falls if neither happened.

  3. Drive motor power inverter module software (Volt)

    Supported by vehicle data
    Why it ranks here
    GM service update 27660 reprograms the drive motor power inverter module on the 2016 Volt for P0AC4 with P0AB9 and a stall, and PIC6294 covers P0AC4 with P2797 on the 2016-2017 Volt and Malibu.
    What would change this
    Rises on a 2016-2017 Volt or Malibu with an open software update. Falls if the software is current.

  4. Poor ground connection at G106 or G113 (Volt, ELR)

    Possible — needs testing
    Why it ranks here
    GM PIP5008A tells technicians to check these ground points when P0AB9 and P0AC4 set on the 2011-2014 Volt and 2014 ELR; the summary does not say how often they are the cause.
    What would change this
    Rises if a technician finds a loose or corroded ground. Falls if the grounds are clean and tight.

What to test first

Start at step 1. Each result tells you what it means and where to go next.

  1. Have all modules scanned and every code and freeze frame saved before anything is cleared. Note the codes stored with P0AC4.

    Low riskTool: OBD-II scan tool that reads hybrid modules

    Warning:Do not clear codes first; GM says the stored data is needed for diagnosis.

    • IfOther hybrid codes such as P0AB9 or P2797 are stored

      It meansWork on those; P0AC4 is the flag.

      Go to step 3

    • IfP0AC4 is the only code

      It meansThink back over how the car was used.

      Go to step 2

  2. Think back: did the fuel run very low, or was the car switched off while driving, shortly before the light came on?

    Low risk

  3. Ask a dealer whether your VIN has an open software update or bulletin for P0AC4.

    Low risk

  4. Have a technician trained in hybrids diagnose the other stored codes and check ground points.

    Best left to a shopTool: manufacturer scan tool

    Warning:Hybrid grounds and modules sit near high-voltage parts. Leave this to a trained technician.

Symptoms

  • Check engine light
  • "Engine Unavailable" or "Engine Not Available" message
  • "Reduced Propulsion" message
  • Engine stalls and will not restart until the key is cycled Gets more serious: stop and have it checked.

Compare with: Check engine light with loss of power

How to fix

This is a safety-critical system, so we explain the code but don't give repair steps. Have it inspected by a qualified technician.

On your vehicle

Chevrolet

  • 2011–2017 · Volt

    Supported by vehicle data

    GM bulletins pair P0AC4 with P0AB9, Engine Unavailable and Reduced Propulsion on 2011-2015 Volts, and with a stall or P2797 on 2016-2017 Volts (inverter reprogram). Status not verified.

  • 2016–2017 · Malibu

    Supported by vehicle data

    GM bulletin PIC6294 lists P0AC4 with P2797 on the Malibu as well as the Volt. Status not verified.

  • PIP5008AStatus unverified

    MIL on, possibly Engine Unavailable, P0AB9 and P0AC4

    2011–2014volt · NHTSA 10111981

    Do not clear codes; confirm the car was not run very low on fuel or keyed off while driving; check grounds G106 and G113.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • PIP5202Status unverified

    Engine Unavailable and Reduced Propulsion with P0AB9 and P0AC4

    2011–2014volt · NHTSA 10112080

    Do not clear codes; update the scan software, read all modules and save freeze-frame data.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • PIP5202AStatus unverified

    Engine Unavailable and Reduced Propulsion with P0AB9 and P0AC4

    2011–2015volt · NHTSA 10114769

    Revision A of PIP5202 with extended model years.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • PIC6294Status unverified

    Diagnose and repair P0AC4 (engine control module) with P2797 (hybrid powertrain control module)

    2016–2017volt, malibu · NHTSA 10114092

    Diagnostic information for these two codes; the NHTSA summary does not give the repair.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • GCUS-3-620Status unverified

    Service update 27660: engine may stall, Engine Not Available

    2016volt · NHTSA 10135428

    Dealers reprogram the drive motor power inverter module for a stall with P0AB9 and P0AC4; the engine will not start until the key is cycled.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

Cadillac

  • 2014–2015 · ELR

    Supported by vehicle data

    GM lists the same P0AB9 and P0AC4 pairing on the ELR, with do-not-clear-codes guidance. Status not verified.

  • PIP5008A (ELR)Status unverified

    MIL on, possibly Engine Unavailable, P0AB9 and P0AC4

    2014elr · NHTSA 10111981

    Same GM bulletin as the Volt entry, filed under the ELR.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • PIP5202 (ELR)Status unverified

    Engine Unavailable and Reduced Propulsion with P0AB9 and P0AC4

    2014elr · NHTSA 10112080

    Same GM bulletin as the Volt entry, filed under the ELR.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • PIP5202A (ELR)Status unverified

    Engine Unavailable and Reduced Propulsion with P0AB9 and P0AC4

    2014–2015elr · NHTSA 10114769

    Revision A of PIP5202 filed under the ELR.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

No curated relationships for P0AC4 yet.

All Hybrid and electric drive codes

P0AC4 questions

Can I clear P0AC4 and keep driving?

Do not clear it first. The stored codes and freeze-frame data show what the hybrid system found, and a stall is possible. Have all modules scanned.

Sources and review

  1. OBD-II code P0AC4 — Kelley Blue Book
  2. GM PIP5202 / PIP5202A (NHTSA 10112080, 10114769) — General Motors
  3. GM Service Update 27660 / GCUS-3-620 (NHTSA 10135428) — General Motors
  4. GM PIC6294 (NHTSA 10114092) — General Motors
  5. GM PIP5008A (NHTSA 10111981) — General Motors
Last reviewed
Written by
Saurabh K.
Editorial review
Pending
Technician review
Technician review pending

This page explains what a code usually means; it is not a substitute for a professional inspection. How we source and review · Report an error

My Car

Pick your car and code pages will point out notes and bulletins for it. Saved only in this browser.

Fill this in from your VIN

Used once to identify your engine; not stored.