If your scan tool displays a DTC with a colon and two hex digits — P0130:11, U0121:87, C1234:2F — those extra characters are the failure type byte (FTB). They appear alongside the base DTC code and tell you exactly what kind of fault the module detected on that circuit or function. Understanding the FTB turns a generic “circuit fault” into a specific fault type before you open the hood.
What the FTB is and where it comes from
The failure type byte is a standardized one-byte (two hex digit) suffix defined in SAE J2012 and ISO 15031-6. When a control module detects a fault condition, it stores not just the base DTC (which identifies what it was monitoring) but a qualifier that describes the nature of the fault. The full fault record is: base DTC + FTB. Some manufacturers expose the FTB in their scan tool displays natively; others require OEM-level tools to access the detailed fault record. Generic OBD-II readers typically show only the base DTC without the FTB suffix.
The same base DTC can appear with different FTBs on the same vehicle at different times — or from different fault events. P0130 (O2 sensor circuit) with FTB :11 means circuit short to ground. P0130 with FTB :15 means circuit open. Both set P0130, but the fault type and the diagnostic approach are completely different.
Common FTB values and what they mean
The SAE J2012 standard defines a common set of FTB values. Not every value is used by every manufacturer, and some OEMs use proprietary FTB codes above the standardized range — but the most common values are consistent across platforms that implement J2012 fully.
| FTB (hex) | Fault type | Typical diagnosis direction |
|---|---|---|
| :00 | No additional information | Work from the base DTC — no specific circuit fault type stored |
| :11 | Short to ground | Signal wire shorted to chassis ground; check harness routing near ground points |
| :12 | Short to battery | Signal wire shorted to supply voltage; check for chafing near power feeds |
| :13 | Open circuit | Break in circuit — check connector, splice, and continuity along harness |
| :14 | Short to ground or open | Either short to ground or open; unplug test separates the two |
| :15 | Short to battery or open | Either short to supply or open circuit; test with sensor unplugged |
| :16 | Circuit voltage below threshold | Supply or signal voltage low — test reference voltage and sensor ground |
| :17 | Circuit voltage above threshold | Signal above expected range — check for open ground or short to power |
| :1C | Circuit current below threshold | Current draw lower than expected — suspect open or high resistance |
| :1D | Circuit current above threshold | Current draw higher than expected — suspect short or failed actuator |
| :2F | Signal erratic | Intermittent fault — check connector tension, backprobe during wiggle test |
| :62 | Signal compare failure | Rationality fault — sensor reads plausible but contradicts another sensor |
| :71 | Actuator stuck low | Actuator commanded high, reads low — mechanical or electrical failure |
| :72 | Actuator stuck high | Actuator commanded low, reads high — mechanical binding or short |
| :87 | Bus signal or message failure | CAN message not received — check bus termination, node power, and ground |
| :96 | Component internal failure | Module self-test failed — internal fault, not wiring |
| :9D | Component or system over-temperature | Thermal protection triggered — check cooling, duty cycle, or load conditions |
Where FTB data shows up in scan tools
Display of the FTB depends on both the scan tool and the vehicle platform. Generic OBD-II tools typically show only the five-character base DTC (e.g., P0130) without the FTB — they read the Mode $03 service which returns the DTC without fault type detail.
OEM tools and capable aftermarket scan tools that access manufacturer-specific diagnostic modes will show the full fault record including the FTB. The format varies:
- VAG (VCDS/ODIS): VCDS shows DTC codes with FTB-equivalent descriptors in a separate column — often displayed as the fault type string rather than the hex byte (e.g., “Short to Ground”). The VAG fault record includes the FTB alongside freeze frame conditions and a priority/maturity counter.
- Toyota/Lexus (Techstream): Techstream exposes sub-code detail in the fault record view. Sub-codes map to the SAE FTB scheme for most powertrain codes.
- Hyundai/Kia (GDS/KDS): FTB appears as a numeric sub-code alongside the main DTC in the stored fault record detail view.
- GM (GDS2/Tech2): GM calls these “failure records” and stores them alongside the DTC with more granular test failure data than the SAE byte alone.
- Generic aftermarket (Autel, Launch, Snap-on): Most advanced aftermarket platforms decode and display the FTB when they access OEM-protocol diagnostic modes rather than generic OBD-II.
How FTB changes the diagnostic approach
The FTB turns a five-step diagnostic procedure into a two-step one in many cases. Without it, you test for every possible fault type on a circuit. With it, you know immediately which type to test for.
Example: P0335 (crankshaft position sensor circuit) stored with FTB :2F (erratic signal) versus :13 (open circuit).
- :13 (open circuit) — check the connector, test continuity of the CKP signal and ground wires back to the PCM, verify the air gap between reluctor and sensor. A static voltage and continuity check is likely sufficient.
- :2F (erratic signal) — the circuit is not open. Static tests will likely pass. The fault is intermittent — connector terminal tension, harness movement under vibration, or a sensor element that intermittently drops out. A scope on the CKP signal while the engine is running and the harness is wiggled will find it. Static testing will not.
The same base DTC, two completely different diagnostic paths. The FTB routes you to the right one immediately.
Similarly, :87 (bus signal or message failure) on a U-code like U0121 tells you immediately that the CAN communication layer is the fault — power, ground, termination, and bus health — rather than an internal module failure (:96). A :87 on a communication code means diagnose the network. A :96 means diagnose the module.
FTB on manufacturer-specific codes
Manufacturer-specific DTCs (B-codes, C-codes, U-codes above the SAE range, and all codes with manufacturer-assigned sub-ranges) use FTBs in the same way, but the FTB values may be OEM-specific rather than SAE-standard. The five-digit hex codes you see in VAG systems — for example, 01314 with sub-type 008 — are a different encoding entirely and should be looked up using OEM documentation or VCDS fault lookup rather than the SAE J2012 table. The principle is identical: the sub-code qualifies the fault type; the base code identifies the function.
How to access FTB data on different scan tools
Not all scan tools expose the FTB equally. Access depends on the tool, the protocol, and the manufacturer’s implementation:
- Generic OBD-II readers: Most budget and mid-range generic readers display only the base DTC. The FTB is stored in the module’s extended fault record but is not surfaced through the standard OBD-II Mode 03 DTC request. To see the FTB with a generic tool, look for a “fault details” or “extended data” option — it is present on some mid-range tools but absent on basic code readers.
- Manufacturer-specific software (VCDS, Techstream, GDS, Forscan, ISTA): These tools typically display the full fault record including the FTB in the DTC detail view. VCDS on VAG platforms is the clearest example — every stored fault shows the base code, the FTB (displayed as a hex suffix or description), the occurrence counter, and the freeze frame. Toyota Techstream similarly shows the sub-type byte for each stored code.
- Professional aftermarket tools (Autel, Launch, Snap-on, Bosch): Mid-to-high-end aftermarket tools with manufacturer-specific coverage display FTB data when the module supports it. The level of detail varies by make and model — coverage for domestic and Japanese platforms is typically good; some Asian and specialist European brands may show only the base code.
If your scan tool does not show the FTB and the code is ambiguous — for example P0130 with no further detail, which could be a short, an open, or a slow response fault — switch to a tool with enhanced access for that platform before starting circuit testing. Testing for the wrong fault type wastes time and risks condemning a component that is functioning correctly.
A real diagnostic example: FTB :13 vs FTB :2F on the same code
Two vehicles arrive with P0335 (CKP sensor circuit). Vehicle one shows P0335:13 — open circuit. Vehicle two shows P0335:2F — signal erratic.
On vehicle one (FTB :13), the diagnostic path is straightforward: the signal is not reaching the PCM. Check the CKP connector for a backed-out pin, measure continuity on the signal wire from the sensor to the PCM, and confirm the sensor ground. The fault is almost certainly in the wiring or connector, not the sensor element itself.
On vehicle two (FTB :2F), the signal is present but intermittent and erratic. This points immediately to: a connector with marginal pin tension that breaks contact under vibration, a hairline crack in the tone ring producing a consistent missing pulse at one rotational position, or a heat-sensitive sensor that changes output when hot. A static circuit check will find nothing — the fault requires live monitoring during reproduction. A scope during a test drive will capture the intermittent dropout that a scan tool PID would miss entirely. See how to diagnose intermittent faults for the monitoring approach.
Same base DTC. Completely different fault type, different diagnostic path, different repair. The FTB made that distinction before any testing began.
Related articles
- How to diagnose intermittent faults
- How to use OBD-II freeze frame data
- Symptom vs DTC: which one should lead your diagnosis
- OBD-II readiness monitors explained
Frequently asked
My generic OBD-II reader does not show the colon suffix. Where does it go?
Generic readers use OBD-II Mode $03 which returns only the five-character base DTC. The FTB is stored in the module’s manufacturer-specific fault record, not in the OBD-II standardized data stream. To read the FTB you need a scan tool that accesses the manufacturer’s own diagnostic protocol — either an OEM tool or a capable aftermarket platform with the correct OEM software for the vehicle.
Can the FTB change between key cycles?
Yes. If the fault is intermittent and manifests differently on different events, the module may store the same base DTC with different FTBs. Some platforms store only the most recent FTB; others accumulate multiple failure records. In VCDS, the fault record shows a counter indicating how many times the fault has set — reviewing multiple records on an intermittent code can reveal whether the fault type is consistent (:2F every time, pointing to an intermittent connection) or variable (different types on different events, pointing to a more complex issue).
Does clearing the DTC also clear the FTB?
Yes. Clearing stored fault codes clears the entire fault record including the FTB. If you need to identify the FTB on a fault that you have not yet diagnosed, record it before clearing. This is particularly important for intermittent faults that may not return under the same conditions — the FTB from the original event can save significant diagnostic time.