5-gas analyzers measure five key exhaust gases: Carbon Monoxide (CO), Carbon Dioxide (CO₂), Hydrocarbons (HC), Oxygen (O₂) and Nitrogen Oxides (NOₓ). Together they paint a comprehensive picture of how efficiently an engine is burning fuel — and what's going wrong when it isn't.
Why five gases?
Traditional MOT testing in the UK focuses on CO, HC and O₂ (with a lambda derivation). That covers the legal basics, but adding CO₂ and NOₓ gives you critical additional context — especially the ratio between CO and CO₂, which is the single most revealing number on the analyzer screen.
Carbon Monoxide (CO)
What it is: an incomplete combustion product. High CO means fuel isn't fully burning.
Typical range at idle: 0.1 – 2.0 %vol
High CO indicates:
- Rich mixture (too much fuel)
- Ignition weakness (misfires burn fuel incompletely)
- Oil burning / carbon contamination
Carbon Dioxide (CO₂)
What it is: the main complete-combustion product. CO₂ peaks when combustion is optimal.
Typical range at idle: 12 – 16 %vol
Low CO₂ indicates:
- Lean mixture (fuel doesn't fully combust)
- Exhaust leaks (ambient air dilutes the sample)
- Probe too shallow (ambient mixing at tailpipe)
// Rule of thumb
CO + CO₂ at idle should be ≥ 12 %vol. Below that, suspect probe placement or exhaust leakage before reading anything else.
Hydrocarbons (HC)
What it is: unburned fuel. High HC means fuel is passing through without igniting.
Typical range at idle: < 100 ppm (well-tuned engine)
- Misfires (no spark, weak coil, fouled plug)
- Injector leaks
- Low compression
Oxygen (O₂)
What it is: unused oxygen in exhaust.
Typical at idle: 0.1 – 1.0 %vol
- Lean mixture (excess air)
- Exhaust leak pre-probe
- Secondary Air Injection still active
Nitrogen Oxides (NOₓ)
What it is: formed at high combustion temperatures. Rises with advanced timing, high load, lean-under-load conditions.
At idle NOₓ is typically very low. It becomes the key gas at high-load tests or under EGR evaluation.
Using gases together — signatures
No single gas tells the full story. The power of 5-gas analysis is in the relationships.
Vacuum leak signature
Rich idle signature
Ignition misfire signature
Getting started — the flow
- Probe placement: 30 cm into the tailpipe. Verify CO + CO₂ ≥ 12%.
- Record at low idle with engine fully warm.
- Calculate theoretical λ (Bretschneider) and compare to measured.
- Match gas signature against the library of known patterns.
- Cross-check with OBD freeze frame, fuel trims and DTCs.
In subsequent articles we dive into Bretschneider's formula, specific fault patterns and how the 4D Petrol Diagnostic Engine automates the pattern match.