Chassis, generic

C0078 Tire Diameter

Check engine lightUsually off
EmissionsNot relevant
Drive cycleRequired
Limp modeNo

Covers a tyre-diameter fault or mismatch detected by the ABS/ESC or indirect-TPMS logic. The system learns the rolling diameter of each tyre from wheel speeds at known vehicle speed; a tyre that is a different size, badly worn, or a mismatched spare makes one wheel turn at a different rate, which the module flags as a diameter problem. The code is set when a learned or compared diameter falls outside the allowed range or the learn routine cannot complete. Indirect-TPMS patents rely on exactly this rolling-radius comparison to infer low pressure, so a true size mismatch can disturb TPMS and stability logic. A fault can affect TPMS and stability functions and sets the warning lamp; correcting tyre sizes and relearning clears it. The entry follows the SAE J2012 generic definition.

Likely causes

Mismatched or wrong-size tyre fitted
Badly worn tyre changing rolling radius
Diameter not relearned after tyre change

Symptoms

  • TPMS or stability warning lamp on
  • Inaccurate speed or TPMS readings
  • Learn routine will not complete

Repair estimate

DifficultyEasyDIY possible
Parts and labour€0–400
Time0.2–1.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

tires

Related codes

C0077C0080C0083

References: patents.google.com, en.wikipedia.org.