Method for Monitoring Sensor Signals and Quantitative Determining of the Stoichiometric Fuel-Air Ratio of the Type of Fuel Used by Means of an Injector Test and Catalyst Diagnosis in a Vehicle

A method for the onboard diagnosis in a vehicle having a catalytic convertor and a lambda-controlled internal combustion engine in the running operation of the vehicle, includes determining the currently maximum possible oxygen storage capacity of the catalytic convertor as well as a measured temporal duration between the lean spike of the pre-catalyst lambda probe and the post-catalyst lambda probe takes place by means of an OSC diagnosis. The method also includes determining a theoretical residual oxygen content and determining a theoretical temporal duration. When the quotient between the measured temporal duration (Δt) and the theoretical temporal duration (Δttheo) lies within a predefined range delimited by a first and a second threshold value (SW1; SW2), thus:SW⁢⁢1≤Δ⁢⁢tΔ⁢⁢ttheo≤SW⁢⁢2,it is determined that the pre-catalyst lambda probe and the post-catalyst lambda probe operate without flaw.