Bretschneider calculates λ from exhaust gases. A wideband O₂ sensor measures λ directly. The two should agree to within ±0.03–0.05 on a healthy system. When they don't, one or both are giving you useful information about a fault.
Which to trust
- Exhaust leak before probe: trust λ_calc. Extra air inflates O₂ and over-reads lean on the sensor.
- Lazy / contaminated sensor: trust λ_calc during transients.
- Probe depth wrong: neither is trustworthy — fix sampling first.
- Extreme HC (> 1000 ppm): λ_calc formula breaks down; trust the sensor if conditions are stable enough.
- Both agree: mixture value is correct to within the margin. Proceed.
Transient vs steady state
Bretschneider assumes steady-state combustion. During rpm sweeps or load changes the formula lags the instantaneous lambda. Compare only at warm idle or steady cruise.
Fuel composition
If the vehicle is running an unknown fuel mix (E10 masquerading as E5, or partially ethanol), λ_calc can drift by ±0.02. Always declare fuel type correctly in the calculator.