Which parameters matter (temperatures, air mass, fuel trims, oxygen sensors, boost), which values are plausible and how to read them step by step.
Live data shows what the engine ECU "sees": temperatures, air flow, fuel trims, pressures. Learn which parameters matter, which values are plausible and how to use them to confirm a cause before replacing parts.
A fault code tells you which test the ECU failed, not which part is faulty. Live data is the evidence: whether a cold engine's coolant temperature equals air temperature, whether air flow rises with revs, whether actual boost follows the command. A simple OBD-II scan tool reads the standard parameters on almost any modern car; the trick is to pick a few parameters, read them under controlled conditions and compare them with each other.
Original editorial schematic, illustrative only — not a real scan-tool screenshot and not a substitute for engine-specific procedures.
Read a few parameters at a time, in repeatable conditions (cold, warm idle, 2,500 rpm), and compare them with each other and with the engine specification. A single number out of context proves nothing.
Live parameters (PIDs in the OBD-II standard) are the values the ECU reads from sensors or calculates: rpm, load, coolant and intake air temperature, air mass, manifold pressure, fuel trims, oxygen sensor voltage, ignition timing.
They turn a code such as P0171 (lean mixture) from a guess into a diagnosis: fuel trims show how lean the engine runs and in which conditions, and air mass shows whether the sensor measures plausibly.
Sources: Wikipedia — OBD-II PIDs (standard live parameters)On a phone, scroll the table to see all columns.
2. The parameters that matter and plausible values| Parameter | What to watch | Sign of a problem |
|---|---|---|
| Coolant temperature | When cold, close to air temperature; when warm, stable at operating temperature | Large gap to air temperature when cold (sensor) or an engine that does not warm up (thermostat, P0128) |
| Fuel trims (STFT / LTFT) | Close to 0 %, moving a few percent | Strongly positive (lean, unmetered air) or strongly negative (rich) |
| Air mass (MAF) | Rises in proportion to rpm and load | Low at idle versus specification, jumps for no reason |
| Upstream oxygen sensor (narrowband) | Switches quickly between about 0.1 and 0.9 V at warm idle | Stuck high, low or switching lazily |
| Requested / actual boost | Actual follows the request under load | Actual stays below (P0299) or overshoots (P0234) |
| System voltage | About 13.8–14.7 V with the engine running, on conventional charging | Close to battery voltage: the alternator is not charging |
Technical note: The values above are indicative. Exact thresholds depend on the engine and come from the manufacturer's documentation.
P0171: system too lean →P0128: thermostat →With the ignition on, read stored and pending codes first. They tell you which parameters are worth watching.
Select 4–6 parameters related to the problem. The more you add, the slower a cheap adapter refreshes and the more fast changes you miss.
With the engine cold for a few hours, coolant and intake air temperature should be close to outside temperature. It is the simplest plausibility test of the temperature sensors.
At operating temperature with loads off, record fuel trims, air mass, oxygen sensors and voltage. This is your baseline.
Hold revs steady for a minute. Trims that normalise at higher revs usually indicate unmetered air at idle; trims that worsen indicate fuel starvation.
For faults that appear only under load, log data or a graph during a drive and analyse it once stopped.
Warning: The driver never watches the phone or scan tool while driving. A passenger reads the data, or you log it and analyse it after stopping.
Compare values with the specification and with each other (for example, air mass against rpm and load). Decide on a part only when the data points the same way in several conditions.
Free: 5 reports a month
Save these diagnostic steps and create a professional technical report with your logo and workshop details to hand to your customer as a PDF: codes read, findings, photos and recommendations. Import the codes from a photo of the scan tool and issue the report in 13 languages, in the customer's currency.
Create the diagnostic PDF reportImport codes from a photoTick off completed checks. The list resets when you reload the page.
0 / 7 checks completed
Codes read before live data Parameters chosen for the problem Cold reading before starting Baseline at warm idle Reading at 2,500 rpm Loaded logging done safely Conclusion based on several conditionsYes, the standard OBD-II parameters. Make-specific parameters (for example, injector corrections on some engines) usually need a scan tool or an app compatible with the make.
What do fuel trims of +15 % mean?The ECU is adding fuel because the oxygen sensor sees a lean mixture. Typical causes are unmetered air, low fuel pressure or an air mass meter under-reading.
Why do values differ between apps?Apps may show different units or averages and refresh at different rates. Always compare with the same app under the same conditions.
Do you offer diagnostics, repairs or vehicle inspections?
Discuss a partnership with OBDAuto.roDiagnostics and OBD codes
Scanners, connection and correct code clearing; live data and freeze-frame explained. Compare ELM327, Bluetooth and professional tools, with official screenshots and criteria for real tests.
Diagnostics and OBD codes Bleeding the cooling system and a stuck thermostat: symptoms and procedureHow to recognise air in the circuit or a stuck thermostat (P0128), how to bleed it properly and which signs point to a head gasket.
Diagnostics and OBD codes Cheap ELM327 scanner: how to use it, which apps are worth it and the risksChips, protocols and adapters compared honestly, apps matched to makes and the clear limits of universal OBD diagnostics.
Diagnostics and OBD codes Dacia Logan II fault codes: symptoms and checksOBD examples for Dacia Logan II, grouped by engine: misfires, mixture, turbo, EGR and DPF. Explanations and diagnostic checks.