C0078Tire Diameter
- Generic OBD-II code · Chassis
- Needs a drive cycle to confirm a fix
- How the car detects it
- 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.
- What you may notice
- TPMS or stability warning lamp on
- Inaccurate speed or TPMS readings
- Learn routine will not complete
- Likely causes
- LikelyMismatched or wrong-size tyre fitted
- PossibleBadly worn tyre changing rolling radius
- RareDiameter not relearned after tyre change
- Repair
- Easy · Often possible to do yourself.Typical cost: €0 – €400
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.