Abstract: The present invention discloses a method for determining the overall efficiency of an automotive exhaust system including a catalytic converter for removing unburned hydrocarbons and carbon monoxide from the exhaust gases. The carbon monoxide, hydrocarbon, and oxygen levels are monitored in the exhaust gases at the tailpipe. In a properly tuned engine, the base oxygen level is ascertained in order to establish the dilution effect of the air injection system on the exhaust gases which dictates the proper working curve on a disclosed Conversion Efficiency v. Carbon Monoxide at Tailpipe graph to be used in the procedure. A source of extra hydrocarbon is injected into the carburetor air intake to enrich the fuel mixture to the engine without affecting the basic carburetor settings. The engine and converter are thus overfueled thereby oxygen starving the converter to minimize any catalytic action. The carbon monoxide level is then read.

Abstract: An improvement to a misfire monitor sensing lean-roll in an internal combustion engine by comparing the rate of change in O.sub.2 in the exhaust gases with respect to time against a limit is disclosed. In the preferred embodiment, the sensed O.sub.2 level in the exhaust is used as an input to rescaling means which automatically normalizes the values used in the comparison to a standard range. Normalization is accomplished alternately by rescaling the rate of change value, rescaling the limit value used, or diluting the exhaust gases sampled to the level of a "standard" air blown engine to which the comparison limit is scaled.

Abstract: Exhaust gas diagnostic apparatus capable of use on both catalytic converter equipped and non-catalytic converter equipped motor vehicles is disclosed. A fast response oxygen analyzer is coupled with means for differentiating the signal output of the oxygen analyzer. In this manner, both the quantity and rate of change of oxygen in the exhaust gas is made available to apparatus included therein for indicating and/or alarming quantity and rate of change values outside preset limits. The apparatus is particularly directed to the detection of an engine misfire condition. The apparatus disclosed permits diagnosing problems without disconnecting the air injector system (if present) or having a port prior to the catalytic converter. As it alarms on transients, it is independent of the steady state oxygen level.