C0078: Tire Diameter (Subfault)

What it means

This code identifies a fault associated with tire diameter information. The specific failure depends on the diagnostic subtype.

At a glance

• Usually lights the ABS or stability-control warning light

How it works

The ABS or stability-control system compares wheel-speed information to estimate whether the tires have compatible rolling diameters. C0078 flags a tire-diameter mismatch or implausible diameter information, which can disrupt wheel-speed comparisons and limit stability functions.

Common causes

Most likely first.

1. Wrong tire size or mismatch

Symptoms

• ABS or stability-control warning light
• Stability control may be limited or disabled

How to diagnose it

1. Read the C0078 subtype and freeze-frame data, and check for wheel-speed or tire-pressure codes that may explain the mismatch.
2. Check the size markings on all four tires and compare them with the vehicle's approved tire sizes.
3. Measure tire pressures and inspect for a significantly worn, undersized, or mismatched tire that could change rolling circumference.
4. Use scan-tool live data to compare the four wheel-speed readings during a controlled, safe low-speed check.
5. If tire sizes and pressures match, inspect the wheel-speed sensor wiring and sensor installation, then test the indicated circuit to the manufacturer's specification.

Common fixes

• Set all tire pressures to the vehicle's specified cold pressures.
• Fit a matching approved tire where a wrong or mismatched size is confirmed.
• Replace a tire with excessive or uneven wear when it creates a diameter mismatch.
• Repair damaged wheel-speed sensor wiring or replace a confirmed faulty sensor.

Can I keep driving?

Avoid relying on ABS or stability control until the cause is corrected, especially on wet or loose surfaces. If braking feels normal, a cautious short trip for repair may be possible, but stop if handling or braking changes.