U0078

Reserved by Document

U0078 is a generic OBD-II network diagnostic trouble code: Reserved by Document. It is logged by the engine control unit when the network monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
U0078
Group
Network
System
Network
Severity
Warning (MIL on)
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What U0078 means

U0078 is stored when a module on the vehicle network, most often the PCM or body control module (BCM), loses serial data communication with the air conditioning control module (ACCM). Modern vehicles integrate the HVAC system with the powertrain and body networks so that the PCM can adjust engine idle speed, compressor load compensation, and evaporator freeze-protection strategies based on signals from the ACCM.

Loss of communication with the ACCM means the PCM may disable A/C compressor clutch engagement as a precaution, or it may no longer receive evaporator temperature data needed to prevent freeze-up. Passenger compartment climate control functions may revert to a fail-safe mode, disabling automatic temperature regulation while preserving manual fan operation in some systems.

Root causes are similar to other U-code faults: damaged CAN or LIN bus wiring, corroded connectors, a blown supply fuse, or a failed ACCM. Because climate control modules are often located behind the dashboard, connector corrosion from minor water leaks is a frequent contributor. Diagnosis should start with power supply and bus integrity verification.

Common causes

Most-frequently reported root causes when U0078 is logged.

  • 1
    Open or short in the CAN or LIN bus segment connecting the ACCM.
  • 2
    Corroded or damaged ACCM harness connector, often from dashboard moisture.
  • 3
    Blown ignition or battery feed fuse for the ACCM.
  • 4
    Failed ACCM internal processor or communication transceiver.
  • 5
    Damaged CAN termination resistor degrading bus signal quality.
  • 6
    Aftermarket radio or HVAC accessory introducing bus interference.
  • 7
    Loose or intermittent ground connection at the ACCM.

Symptoms drivers notice

A/C compressor may not engage or climate control enters fail-safe mode.
Automatic temperature regulation inoperative; manual fan control may still work.
MIL or HVAC warning indicator illuminated.
PCM cannot perform idle-up compensation for compressor load.
Multiple U-codes may be present if the issue is bus-wide.

How to diagnose U0078

A typical diagnostic flow when this code is present.

  1. 1
    Scan all modules and document all DTCs to distinguish isolated ACCM loss from a broader network fault.
  2. 2
    Verify ACCM power supply voltage and ground circuit integrity.
  3. 3
    Inspect the ACCM connector behind the dashboard for moisture, corrosion, or bent pins.
  4. 4
    Attempt to communicate with the ACCM directly using a scan tool module scan function.
  5. 5
    Check the relevant CAN or LIN bus wiring between the ACCM and gateway or BCM.
  6. 6
    Clear codes and attempt to reproduce the fault; intermittent faults often point to connector issues.
  7. 7
    Replace the ACCM only after ruling out all wiring and power supply faults.

Related network codes

Frequently asked questions

Will U0078 prevent the A/C from working at all?

It depends on system design. Many vehicles disable automatic A/C or compressor engagement when the ACCM is unreachable, but manual blower control often remains functional.

Can a software update fix U0078?

If the code results from a known firmware incompatibility between modules after a software update, reprogramming the ACCM or another module may resolve it. However, hardware faults must be ruled out first.

Is U0078 dangerous to drive with?

U0078 is generally not a safety-critical fault; however, loss of climate control visibility (fogging) in cold, wet conditions could indirectly affect safety.

Could an aftermarket stereo cause U0078?

Yes. Aftermarket head units that tap into the vehicle's data bus without proper isolation can introduce bus loading or interference that causes modules to drop communication.

Disabling U0078 in software

RaceTune can permanently disable U0078 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

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