OBD diagnostics · a working order
Car diagnostics: start with evidence
A scan tool reports what a vehicle's control unit detected. It does not tell you which part to buy. Use the code, the conditions in which it appeared and a targeted test to narrow the fault before authorising a repair.
1 — Assess the warning before driving or testing
A steady check-engine light calls for diagnosis soon. A flashing check-engine light can signal a serious misfire: reduce load, find a safe place to stop and seek repair help promptly. If the vehicle also has an oil-pressure or temperature warning, smoke, overheating or a major change in how it drives, follow the owner's manual and arrange assistance. Do not perform a loaded road test to collect data from a vehicle that may be unsafe.
2 — Save the evidence before clearing anything
Record the vehicle, engine, mileage, symptoms and when the problem occurs. Read all available modules with a suitable scan tool and save stored, pending and manufacturer-specific codes where available. Keep freeze-frame data, which can show operating conditions captured around a fault, plus relevant live data. Tool and vehicle support vary.
Do not clear codes just to see whether the light returns: that can remove useful context and reset readiness monitors. If a workshop needs to reproduce the fault, tell them whether the light is intermittent and what changed immediately before it appeared.
3 — Translate the code into a testable question
Read the code's definition, then ask which measurement or monitored system led the ECU to set it. A code can describe a result without identifying its cause. For example, P0299 indicates an underboost condition; it does not by itself prove a bad turbocharger. Companion codes, engine design and freeze-frame conditions help decide which branch to investigate first.
Use our fault-code lookup for a definition, then the in-depth code guides where we have a reviewed diagnostic order. The lookup covers many codes; detailed procedures currently cover a focused boost, air and fuelling cluster.
4 — Measure the system instead of guessing a part
Choose a test that can distinguish competing explanations. For a boost complaint, compare requested and actual pressure in the same units and pressure reference during a valid commanded region. Then use the engine's control architecture, inspection and manufacturer procedure to decide what to test next. A pressure gap is a finding; a leak, actuator or sensor fault is a hypothesis until tested.
Our requested-versus-actual boost guide shows how to read the trace, and the browser-based analyser can describe its shape. It cannot confirm a failed component from the log alone. Work involving hot, moving or pressurised parts belongs with a qualified technician.
5 — Verify the repair under comparable conditions
After a confirmed fault is repaired, repeat the relevant measurement or test under comparable conditions, check that the symptom is gone and scan for returning codes. Record what was tested, what changed and what remains uncertain. Clearing a warning light alone is not evidence that the underlying problem was fixed.
This is a general diagnostic workflow, not a vehicle-specific repair procedure or workshop sign-off. For the distinction between observations and diagnoses, read our methodology.
Background: US EPA guidance on check-engine warnings and OBD diagnosis. Follow the vehicle manufacturer's instructions for the specific car.