Q: Is it safe to drive with C0078 active?

Check for other symptoms. If warning light is on, get diagnosed soon.

Q: How much does it cost to fix C0078?

Repair costs typically range $Varies by component. DIY diagnosis with a multimeter can save on labor.

Q: Will the warning light go off on its own?

If the fault is intermittent, the light may turn off after several drive cycles. However, the underlying issue should still be addressed. ---

This diagnostic article is an original guide created by AIscanAuto for educational and troubleshooting purposes. The technical information about OBD2 DTC codes is based on publicly available automotive diagnostic standards and industry best practices. External links provided are for additional reading and context only — they do not imply endorsement or ownership of third-party content. All repair procedures should be performed by qualified technicians.

DTC C0078: C0078 — Tire Diameter (Subfault): Causes, Symptoms & Fix

The C0078 OBD2 fault code relates to the Tire Diameter (Subfault) circuit. This is part of the vehicle's chassis control system. When a circuit failure occurs, the control module cannot properly monitor or regulate this system, which may affect braking, traction, stability, or suspension performance. At AIscanAuto, we help drivers quickly identify OBD2 trouble codes, understand their root causes, and find the right fix — with AI-powered diagnostics that go beyond basic code reading.

C0078 — Tire Diameter (Subfault): Causes, Symptoms & Fix

The C0078 OBD2 fault code relates to the Tire Diameter (Subfault) circuit. This is part of the vehicle's chassis control system. When a circuit failure occurs, the control module cannot properly monitor or regulate this system, which may affect braking, traction, stability, or suspension performance.

At AIscanAuto, we help drivers quickly identify OBD2 trouble codes, understand their root causes, and find the right fix — with AI-powered diagnostics that go beyond basic code reading.

Symptoms

Chassis warning light
Specific symptoms vary by manufacturer
May be accompanied by other DTCs

Common Causes

Faulty component — internal damage or wear affecting the Tire Diameter (Subfault)
Wiring issues — broken, corroded, or chafed wires in the circuit
Connector problems — loose, oxidized, or moisture-damaged pins
a general circuit failure is detected — can result from wiring faults, connector issues, or component failure
Control module fault — rare, but the chassis control module may have an internal fault

Step-by-Step Diagnosis

Scan with OBD2 tool — Confirm C0078 is active. Check for additional chassis DTCs.
Visual inspection — Examine the Tire Diameter (Subfault) wiring and connector for corrosion or damage.
Check voltage and ground — Use a multimeter to verify proper reference voltage.
Test component — Compare readings to factory specs. A stuck or erratic signal confirms failure.
Check for shorts/opens — Perform continuity tests on the wiring between the component and control module.

Repair & Solutions

If the component is defective, replace the Tire Diameter (Subfault) with an OEM or quality aftermarket part.
Repair any damaged wiring or connectors using proper methods.
Clear the DTC with an OBD2 scanner and test drive to verify the code does not return.
If the code persists after component replacement, inspect the control module connector or consider module testing.

Related Codes

C0075 — Extended Brake Pedal Travel Output to PCM (Subfault)
C0076 — PWM for Traction Control (Subfault)
C0077 — Low Tire Pressure (Subfault)

DTC C0078: C0078 — Tire Diameter (Subfault): Causes, Symptoms & Fix

C0078 Diagnostic Trouble Code

DTC C0078: C0078 — Tire Diameter (Subfault): Causes, Symptoms & Fix

C0078 — Tire Diameter (Subfault): Causes, Symptoms & Fix

Symptoms

Common Causes

Step-by-Step Diagnosis

Repair & Solutions

Related Codes

External References & Code-Specific Resources