C0078 is a chassis code indicating the vehicle's ABS or stability control system has detected an invalid or mismatched tire diameter. The system calculates tire diameter from wheel speed sensor data and sets this code when the calculated diameter differs significantly between wheels or doesn't match expected values.
Different size tires installed on vehicle (e.g., different brands, models, or sizes on front vs rear)
One or more tires severely underinflated or overinflated compared to others, causing different effective diameters
Tires with significantly different wear patterns (e.g., new tires on one axle, worn tires on other)
Faulty wheel speed sensor sending incorrect speed data, causing diameter calculation error
Wheels of different diameters or offset installed on the vehicle
Rarely, the ABS module may have software issue or incorrect tire size configuration (least likely cause)
No. The PCM/ECM is almost never the cause of C0078. The code comes from the ABS/stability control module, which is a separate system. Replacing the engine computer for this code is almost always unnecessary and costly.
Vehicle is typically drivable but traction control and ABS may be disabled or limited. This can affect safety in adverse conditions. Address the cause promptly.
Not necessarily. If the cause is mismatched sizes or pressures, correcting those issues (matching tire sizes, equalizing pressure) often clears the code. Only replace tires if they are worn, damaged, or the wrong size for the vehicle.
Some may recommend this as a first step due to unfamiliarity with the code's common causes. The ABS module itself fails rarely—always rule out tire size, pressure, sensor, and wear issues first before considering module replacement.