4D Petrol Diagnostic Engine — User Manual
The 4D engine reads up to four evidence channels — 5-gas measurements, OBD live data, DTCs with freeze frame, and vehicle identity (VIN) — and returns a ranked diagnosis. You choose which channels to supply; the engine adapts to what it gets.
Open the 4D Engine ↗Six steps to a diagnosis
- Open the engine at 5-gasanalyzermot.com.
- Tick your modules in the left sidebar — GASLab, OBDLab, DTCLab, DNA/VIN. Each ticked box opens that module's panels and sends its channel to the engine. Untick what you don't have; the engine works with any combination.
- Optional — enter the VIN (17 characters). A successful decode identifies the engine and shows only the fields that apply to that car. No VIN? The app drops to a safe basic mode.
- Fill in what you measured. At minimum, the low-idle gas row: CO, CO₂, HC, O₂ — plus NOₓ and lambda on a 5-gas analyser.
- Run the diagnosis.
- Read the result: a ranked top-3 of fault candidates with confidence per family — or an honest abstain when the evidence isn't enough to name a fault. Abstain is a verdict, not an error.
The modules in detail
GASLab — the 5-gas channels
Two panels: Low Idle and High Idle (~2500 rpm). Each takes six fields: CO (%), CO₂ (%), HC (ppm), O₂ (%), NOₓ (ppm) and lambda from the analyser. On a 4-gas analyser, leave NOₓ and analyser-lambda off — the engine suppresses them cleanly. Warm the engine fully and insert the probe at least 30 cm into the tailpipe: at idle, CO + CO₂ below ~12% usually means a dilution / probe-depth problem, and the app will warn you. The high-idle row is optional but valuable — several fault signatures only separate between idle and 2500 rpm.
OBDLab — live PIDs
Low- and high-idle panels for scan-tool live data: engine RPM, coolant temperature (ECT), calculated load, short- and long-term fuel trims (STFT/LTFT, both banks where the car has them), MAF, MAP, and upstream/downstream O₂ sensor readings. With a decoded VIN the app shows only the PIDs your engine actually has (banks, sensor type); without a VIN it shows a safe Bank-1 minimum. Fuel trims are the engine's strongest OBD evidence — enter them at stable warm idle.
DTCLab — codes, freeze frame, Mode $06
Three panels. DTC: current and pending trouble codes plus MIL status. Freeze frame: the 12 mandatory PIDs captured at the moment a code set (RPM, load, ECT, trims, MAP, speed …) with optional extended PIDs behind a toggle. Mode $06: on-board monitor test results, entered per monitor row (test value against its min/max limits) — useful for catalyst and O₂ sensor verdicts before a code ever sets.
DNA / VIN — vehicle identity
Enter the 17-character VIN and the decoder resolves make, engine code, displacement and induction. A high-confidence decode gates the whole UI — fields that don't exist on your engine disappear, and the engine's expectations (trim bands, sensor layout) adapt. A partial or failed decode is not a problem: the app falls back to basic mode and the diagnosis still runs on whatever channels you supplied.
Reading the result
The verdict is a ranked differential: up to three fault candidates across a 12-family fault atlas (rich mixture, lean, misfire, sensor, ECU/electrical, catalyst, exhaust leak, ignition, mechanical wear, cam timing, non-start, no-fault), each with a confidence rating and the evidence layers that drove it. When channels genuinely conflict or evidence is too thin, the engine abstains and says why — by design it refuses to guess. Give it another channel (high-idle gas, trims, freeze frame) and run again; more evidence narrows the differential.
- Engine fully warm, all consumers (A/C, lights) off, stable idle before sampling.
- Probe ≥ 30 cm in; check CO + CO₂ > 12% at idle before trusting the numbers.
- More channels beat more precision — a rough trim reading helps more than a third decimal on CO.
- Take the gas rows and the OBD rows in the same session; mixed-session data can contradict itself and cause an abstain.