21 technical articles
From Bretschneider's lambda formula to the Holy Grail graph — deep dives on the science and the practice of 5-gas analysis.
Read the blog →A diagnostic library for 5-gas analysis
Most online emissions content is surface-level. Here you'll find the math behind lambda, the exact gas signatures that identify specific faults, and the reasoning behind each diagnostic pattern — written for technicians, not beginners.
4D Petrol Engine
Feed it any combination of four channels — 5-gas readings, OBD live data, DTCs with freeze frame, and VIN — and the engine returns a ranked top-3 diagnosis from a 12-family fault atlas, with an honest abstain when evidence is thin.
Launch tool ↗ Read the manual →Gas signatures
Recognize vacuum leaks, rich idle, catalyst failure, ignition misfire and more by their characteristic CO / CO₂ / HC / O₂ / NOₓ signatures.
Learn the method →
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// quick start
60 SEC
Using the 4D engine
Tick the modules you have data for — GASLab, OBDLab, DTCLab, DNA/VIN — fill in the readings, and run. You get a ranked top-3 diagnosis with confidence; insufficient evidence returns an honest abstain, not a guess.
Read the user manual →
// Start here
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Foundational articles
5-Gas Analysis Basics: Understanding CO, CO₂, HC, O₂, NOₓ
What each gas tells you about combustion efficiency — and how they combine to reveal engine faults.
read →Understanding the Bretschneider Lambda Formula
How theoretical lambda is derived from exhaust gases — and why it's often more reliable than your O₂ sensor.
read →Probe Placement Matters: Avoiding False Results
Why depth affects CO + CO₂ totals and how to guarantee accurate measurements every time.
read →Diagnose a Vacuum Leak from the Gas Signature
High O₂, high lambda, low CO/CO₂ — and the confirmation tests that remove all doubt.
read →Catalyst Efficiency Testing: A Practical Guide
Conversion percentages, efficiency bar readings, and the thresholds that separate pass from fail.
read →The Holy Grail Graph: Spotting Leaks Visually
Plot lambda vs RPM to see leaks, EGR issues and anomalies that hide at idle.
read →
// why five gases
REF
The case for full 5-gas
Traditional MOT testing (CO + HC + lambda) misses combustion stories that only become clear with CO₂ and NOₓ in the picture. Each gas is a witness; the signature is the testimony.
incomplete burn
burn completeness
unburnt fuel
lean / leak
combustion temp