The Lambda probe (λ) is the sensor that measures the concentration of oxygen in the exhaust gases. The Lambda value expresses the air/fuel ratio:
- λ = 1.0: stoichiometric (14.7:1 petrol, 14.5:1 diesel)
- λ < 1.0: rich (more fuel)
- λ > 1.0: lean (more air)
Types of probes
- Binary probe (vintage): on/off signal, measures only the presence of oxygen
- Broadband probe (LSU 4.9 Bosch): precise measurement, range λ 0.7-1.8
Lambda in chiptuning
Stage 1 petrol typically works in enrichment (λ 0.85-0.90) at high speeds for thermal protection. Changing the Lambda map is one of the main parameters of petrol tuning.
Lambda Off
When the catalyst is removed (track use), the downstream Lambda probe (post-cat) must be disabled via software to avoid DTCs. Illegal for road use.
Upstream and downstream probes
Modern cars are fitted with at least two probes: one upstream of the catalyst, used for real-time adjustment of the air/fuel ratio, and one downstream, which checks the efficiency of the catalyst. The broadband probe (e.g. Bosch LSU 4.9) measures precisely over a wide range, essential for modern injection strategies.
Lambda in tuning
On turbo petrol engines, Stage 1 carefully manages the lambda map, maintaining a slight enrichment at high loads to thermally protect the turbo and catalyst. Too lean a value at high load is dangerous (detonation, overtemperatures). The deactivation of the downstream probe, linked to the removal of the catalyst, only concerns use on the track and is illegal on the road.
Safety in petrol tuning
The management of the air/fuel ratio is a safety parameter, not just a performance one: a mixture that is too lean at high load, combined with excessive advance, leads to detonation, which can damage the pistons and turbo. For this reason, good petrol tuning maintains prudent enrichment and advance margins, verifying the real behavior on the bench. Tampering with the downstream probe to hide a removed catalyst is only permitted for track use.