Mismatched tire sizes or uneven tire wear across the vehicle; Incorrect tire pressure causing a difference in rolling circumference; Faulty wheel speed sensor or damaged sensor wiring at one wheel; Contaminated or damaged wheel speed sensor tone ring; ABS control module software issue or incorrect tire size calibration; Mechanical fault such as a dragging brake or wheel bearing play affecting wheel speed

DTC codes / C00xx · Brakes, steering and suspension / C0078

C0078Tire Diameter

Q: What does code C0078 mean?

Tire Diameter. C0078 is a generic chassis code indicating that the anti-lock brake system (ABS) control module has detected a mismatch in tire diameter or rolling circumference among the wheels. The module compares wheel speed signals from each wheel speed sensor and flags this code when one or more wheels are rotating at a different rate than expected for the vehicle's programmed tire size, which can be caused by incorrect tire size, uneven wear, or a faulty wheel speed sensor.

C0078 is a generic chassis code indicating that the anti-lock brake system (ABS) control module has detected a mismatch in tire diameter or rolling circumference among the wheels. The module compares wheel speed signals from each wheel speed sensor and flags this code when one or more wheels are rotating at a different rate than expected for the vehicle's programmed tire size, which can be caused by incorrect tire size, uneven wear, or a faulty wheel speed sensor.

SystemChassis

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaBrakes, steering and suspension

Symptoms you may notice

ABS warning light illuminated on the dashboard
Traction control or stability control warning light may also be on
Often no symptoms besides the warning light
Possible intermittent ABS activation or pulsation at low speeds
Speedometer may read inaccurately if the diameter mismatch is significant
Cruise control may be disabled or function erratically

Common causes, most likely first

Mismatched tire sizes or uneven tire wear across the vehicle
Incorrect tire pressure causing a difference in rolling circumference
Faulty wheel speed sensor or damaged sensor wiring at one wheel
Contaminated or damaged wheel speed sensor tone ring
ABS control module software issue or incorrect tire size calibration
Mechanical fault such as a dragging brake or wheel bearing play affecting wheel speed

How to diagnose and fix it

Verify all four tires are the same size, brand, and have similar tread depth; replace or rotate tires as needed
Check and adjust tire pressures to the manufacturer's specification
Inspect each wheel speed sensor and its wiring for damage, corrosion, or loose connections
Clean or replace a contaminated or damaged tone ring
Use a scan tool to view live wheel speed data from all four wheels and compare readings while driving
Repair any mechanical issue such as a dragging brake or worn wheel bearing that affects wheel speed
Clear the code and perform a road test to confirm the repair; if the code returns, consult a dealer for possible module recalibration

Tire Diameter Code Isn't Always a Tire Problem

A common oversight is replacing just one or two tires, which creates a diameter difference that the ABS module interprets as a fault even though all tires are the correct size.
Wheel speed sensor signal dropout from a corroded connector can mimic a tire diameter mismatch, so always inspect wiring before replacing tires.
Some vehicles require a tire size calibration or relearn procedure after tire replacement, and skipping this step will cause the code to reset.
A worn wheel bearing can alter the air gap between the sensor and tone ring, producing an erratic signal that sets C0078 without any tire issue.
This code does not always indicate a safety hazard; it can be set by a temporary low tire pressure condition that has since been corrected.

Can I keep driving with C0078?

You can generally continue driving with C0078, but avoid hard braking or aggressive maneuvers because ABS and stability control may be compromised. Have the tire sizes and pressures checked promptly, and seek service if the warning light remains on after correction.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.