Hard Fault

📖 YOUCANIC Automotive Glossary

A hard fault is a malfunction that is consistently present and easily reproducible every time the affected system is operated or tested. The fault condition exists continuously — every time you start the engine, the misfire is there; every time you read codes, the same DTC reappears immediately; every time you measure the sensor, it reads out of range. Hard faults generate present/current DTCs that remain active and are the easiest type to diagnose because you can test and measure the problem in real time with live data, active tests, and physical measurements. The cause is typically a completely failed component, a fully broken wire, or a permanently stuck mechanism.

Hard faults contrast with soft faults and intermittent faults, which come and go. When approaching any diagnosis, first determine if the fault is hard (always present) or intermittent (sometimes present). For hard faults, follow the standard diagnostic approach: read the DTC, check freeze frame data, monitor related live data PIDs, perform component testing, and inspect wiring and connections. The YOUCANIC UCAN-II clearly indicates whether a code is present/current (hard fault) or stored/history (was present but may now be intermittent).