03 01 00 2FActive, confirmed misfire
P0301-00 with status 0x2F: testFailed, testFailedThisOperationCycle, pending, confirmed, and failed since last clear are set, so the fault is currently failing and confirmed.
UDS Diagnostic Tool
DTC Decoder
Turn the raw bytes of a DTC record into a readable code, failure type, and a bit-by-bit explanation of the DTC status byte.
- Domain
- Vehicle Diagnostics
- Technology
- DTC
- Execution
- Local deterministic lookup
Interactive utility
Decode a DTC and its status byte
Paste the three DTC bytes from a ReadDTCInformation (0x19) response, optionally followed by the status byte, or enter a display code such as P0301-00. The DTC is split using the SAE J2012-DA style layout: system letter, code digits, and failure type byte. The status byte is decoded against the eight ISO 14229-1 status bits, with a plain-language summary and checks for bit combinations that normally exclude each other. Failure type names cover a curated subset; OEM-specific meanings are never guessed.
Ready
Enter a DTC, a status byte, or both
The decoder splits the three DTC bytes into system, code, and failure type, and explains every status bit.
Examples
Decodes the SAE J2012-DA style 3-byte layout and the ISO 14229-1 status byte. Failure type names are a curated subset; OEM-specific meanings are not guessed.
Three-step lookup
How to use the decoder
- 01
Enter the DTC bytes
Enter three hex bytes such as 03 01 00, four bytes with the status byte appended, or a display code with its failure type such as P0301-00.
- 02
Add the status byte
Enter the status byte separately if it is not already the fourth byte. A status byte can also be decoded on its own without a DTC.
- 03
Read the breakdown
Review the system, code group, failure type, the overall fault state, and each status bit before planning the next diagnostic step.
Examples
Common lookup patterns
C1 00 87 · 08Stored network fault
U0100-87 (missing message) with only confirmedDTC set: the fault was confirmed earlier but is not failing in the most recent test.
0x50Status byte only
testNotCompletedSinceLastClear and testNotCompletedThisOperationCycle are set: the monitor has not run since the DTC memory was cleared, so the fault state is not established.
Protocol context
Technical notes
- In the 3-byte format the first two bytes form the SAE-style code: bits 7–6 of the high byte select P, C, B, or U, bits 5–4 give the first digit, and the remaining nibbles give the next three hex digits. The third byte is the failure type byte.
- The layout assumes the SAE J2012-DA DTC format used by most OEMs. Check the DTCFormatIdentifier returned by ReadDTCInformation sub-function 0x01 before relying on the code interpretation.
- Status bits that an ECU does not support always read 0. Compare the status byte with the DTCStatusAvailabilityMask in the same response before treating a clear bit as meaningful.
- A DTC records a detected fault condition. If the ReadDTCInformation request itself fails, the ECU returns a negative response code, which the UDS NRC Decoder explains.
FAQ
DTC and status byte questions
- What is the difference between pendingDTC and confirmedDTC?
- pendingDTC means the test failed in the current or previous operation cycle. confirmedDTC means the fault met the ECU's confirmation criteria and is stored in non-volatile memory until it ages out or is cleared.
- Why does a cleared DTC come back immediately?
- If testFailed is set again right after a clear, the monitor is still detecting the fault. If only testNotCompleted bits are set, the monitor simply has not run yet. The DTC keeps returning troubleshooting guide covers the investigation.
- Is the failure type byte the same across all manufacturers?
- The SAE J2012-DA failure type categories are widely used, but manufacturers can define their own values, especially in the 0xF0–0xFF range. Unlisted values are reported by category only.
- Is a DTC the same as a UDS NRC?
- No. A DTC describes a fault condition stored by the ECU. An NRC explains why a diagnostic request was rejected. Use the UDS NRC Decoder for negative responses.
Diagnostic support
Need help with a DTC that will not stay cleared?
If a DTC keeps returning, sets without an obvious cause, or behaves differently across ECUs, I can help analyze the monitor conditions, status transitions, and supporting traces.