Look closely at the next scan you run, because 35 percent of all diagnostics carrying a fault code also carry an extra two digits that most lookup tools quietly throw away. We looked at our own platform data between February 18 and September 6, 2026, analyzing 1601 diagnostics that included at least one readable fault code, and found that 920 of them contained these extra bytes.
What the extra digits actually mean
A failure-type byte is an extra pair of characters appended to a standard fault code. Instead of seeing P0299, a technician might see P029900. While the base code tells you which component or system is logging an issue, the appended byte specifies the exact nature of the failure. It tells you whether you are dealing with an open circuit, a short to ground, a signal stuck high, or an implausible value.
The problem is that most code lookup tools strip or ignore these extra characters entirely. When that happens, you lose the difference between a mechanical failure and a wiring issue. Any analysis that does not capture the full string ends up double-counting the same underlying code, leaving the technician to guess the testing path.
The most common failure types we see
When we broke down those 920 diagnostics, the appended bytes fell into several distinct categories. The General family made up 59 percent of all byte-carrying codes. Circuit and electrical faults accounted for 12 percent, while Manufacturer-defined and Performance issues each sat at 5 percent.
Here is exactly how the most frequent failure types broke down in our data:
| Byte | Definition | Occurrences | Frequency | Most common code |
|---|---|---|---|---|
| 00 | No sub-type information | 477 | 52% | P0299 |
| 13 | Circuit open | 36 | 4% | C1083 |
| 68 | Event information | 20 | 2% | P250F |
| 87 | Standardised (group not decoded here) | 18 | 2% | U0064 |
| 92 | Performance or incorrect operation | 18 | 2% | P0017 |
| 07 | Mechanical failure | 15 | 2% | C0037 |
| 01 | General electrical failure | 14 | 2% | B1455 |
| 15 | Circuit short to battery or open | 13 | 1% | P0647 |
| 02 | General signal failure | 12 | 1% | P2452 |
| 11 | Circuit shorted to ground | 12 | 1% | B100B |
Why throwing this data away hurts the shop
More than half the time, the byte is 00, meaning no sub-type information is provided. We saw this 477 times, most frequently attached to P0299. However, the other half of the time, the vehicle is literally telling you how the circuit failed.
Take byte 13, which indicates an open circuit. We saw this 36 times in the Circuit and electrical family, most often on C1083. If your tool strips the 13, you might spend an hour testing components when the code is already screaming that the wire is broken. The same applies to byte 11 (shorted to ground, most often on B100B) and byte 15 (short to battery or open, most often on P0647). A mechanical failure byte like 07 (seen on C0037) directs you away from the multimeter and toward the physical part.
At Wrenchlane, the diagnostic platform we build retains and analyzes these bytes. When a scan tool reads a full code, dropping the last two digits is dropping the diagnosis.
Stop guessing at the base code
Next time a vehicle rolls into the bay, look at the full string of characters on the scanner before punching the base code into a generic search bar. If you see those extra two digits, use a tool that actually reads them. Are you seeing similar patterns with appended bytes in your own bays?