C0078 code meaning
Tire Diameter
What C0078 means
Covers a tire-diameter fault or mismatch detected by the ABS/ESC or indirect-TPMS logic. The system learns the rolling diameter of each tire from wheel speeds at known vehicle speed; a tire 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 tire sizes and relearning clears it. The entry follows the SAE J2012 generic definition.
C0078 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modeNot expected
- Repair difficultyEasy, DIY possible
- Typical labor0.2 to 1.5 hours
- Clears after repairAfter a drive cycle
Symptoms of C0078
- TPMS or stability warning lamp on
- Inaccurate speed or TPMS readings
- Learn routine will not complete
Common causes of C0078
- Most likelyMismatched or wrong-size tire fitted
- PossibleBadly worn tire changing rolling radius
- Less likelyDiameter not relearned after tire change
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Tires
What each part of C0078 means
| C | Chassis system: brakes, ABS, steering and suspension. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 78 | The exact fault in that area. |
What to do next
- Note C0078 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about C0078
What does C0078 mean?
C0078 means "Tire Diameter". Covers a tire-diameter fault or mismatch detected by the ABS/ESC or indirect-TPMS logic. It is a generic code, so it means the same on every make.
Is it safe to drive with C0078?
C0078 does not normally turn on the check engine light by itself. Watch for the symptoms above and have it checked soon.
Will C0078 fail an emissions test?
C0078 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is C0078 to fix?
Difficulty: easy. A typical repair takes 0.2 to 1.5 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will C0078 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.