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 might be going wrong.
Why Five Gases?
Traditional MOT testing in the UK focuses on CO, HC, and O₂ (with lambda). While that covers the basics, adding CO₂ and NOx gives you critical additional context:
Carbon Monoxide (CO)
What it is: Incomplete combustion product. High CO means fuel isn't fully burning.
Typical range at idle: 0.1% - 2%
High CO indicates:
- Rich mixture (too much fuel)
- Ignition weakness (misfires burn fuel incompletely)
- Carbon contamination (oil burning, poor combustion)
Carbon Dioxide (CO₂)
What it is: Complete combustion product. CO₂ peaks when combustion is optimal.
Typical range at idle: 12% - 16%
Low CO₂ indicates:
- Lean mixture (not enough fuel burns completely)
- Exhaust leaks (dilution lowers CO₂)
- Probe not inserted deep enough (sampling ambient air)
The CO + CO₂ Total: At idle, this sum should be at least 12%. Values below 12% signal poor probe placement or severe dilution.
Hydrocarbons (HC)
What it is: Unburned fuel molecules. High HC means fuel is passing through without igniting.
Typical range at idle: < 100 ppm (well-tuned engine)
High HC indicates:
- Misfires (no spark, weak coil, bad plug)
- Fuel injector leaks (rich and high HC can both occur)
- Low compression (air-fuel mixture not compressed enough to ignite)
Oxygen (O₂)
What it is: Unused oxygen in exhaust. Indicates how lean or rich the mixture is beyond the lambda sensor's range.
Typical range at idle: 0.1% - 1% (petrol engines)
High O₂ indicates:
- Lean mixture (excess oxygen not burned)
- Exhaust leaks before the probe (oxygen sneaks in)
- Air injection systems (SAI) adding oxygen
Nitrogen Oxides (NOx)
What it is: Formed at high combustion temperatures. NOx rises with advanced timing, high load, and lean mixtures under load.
Typical at idle: Very low or zero (NOx forms at higher temperatures)
High NOx indicates:
- Advanced ignition timing
- Lean burn conditions under load
- EGR not functioning (NOx reduction via exhaust gas recirculation)
- Carbonless combustion (high NOx with low CO)
Using the Gases Together
No single gas tells the whole story. The power of 5-gas analysis comes from examining the relationships between gases:
Example Signature: Vacuum Leak
- O₂: High (5%+)
- Lambda: High (>1.3)
- CO & CO₂: Low (because extra air further unloads the mixture)
- HC: May be elevated if idle quality suffers
Example Signature: Rich Idle
- CO: High (>2%)
- CO₂: May be moderately high (fuel burning incompletely can still produce CO₂)
- O₂: Very low or zero
- Lambda: Low (<0.9)
Example Signature: Ignition Misfire
- HC: Very high (>1000 ppm)
- CO: Often elevated (partial burns)
- O₂: May be normal or slightly high
Getting Started
When you approach a vehicle, follow this process:
- Check probe placement: Insert the probe 30cm into the tailpipe. If CO+CO₂ < 12%, it's not in deep enough.
- Record the 5 gases at low idle: Ensure engine is fully warmed.
- Calculate theoretical lambda: Use Bretschneider's formula (or our 4D tool) to see if the measured lambda matches expected values.
- Look for gas signatures: Compare your readings to known patterns.
- Add OBD data if available: DTCs, freeze frame, and live PIDs help confirm your hypothesis.
In subsequent articles, we'll explore specific fault patterns, the Bretschneider formula, and how to use the 4D diagnostic engine to automate pattern matching.