Methods are disclosed for determining an oxidation state of a catalyst using RF signals. The method may include introducing radio-frequency signals into a resonant chamber including a catalyst, modulating an air-fuel ratio of an engine upstream of the catalyst to generate a sequence of uniform pulses and at least one altered pulse that differs from the uniform pulses, and comparing a frequency response of two or more resonant modes of the radio-frequency signals during the sequence to determine an oxidation state of the catalyst. The method may further include adjusting the air-fuel ratio based on the comparing step. Two or more altered pulses may be inserted into the air-fuel ratio sequence. The altered pulse may have a pulse width and/or amplitude that differs from the uniform pulses. The methods may be used to adjust an air-fuel ratio to correct or impart a bias.

Methods and systems are provided for detecting an air-fuel imbalance based on output from multiple degradation monitors. In one example, a method comprises, during feedback engine air-fuel ratio control responsive to output of an exhaust gas sensor positioned downstream of a catalyst, indicating a cylinder imbalance responsive to a catalyst transfer function determined only within a specified frequency range based on the exhaust gas sensor output after determining that the catalyst is nominal, and adjusting an actuator in response to the indicated cylinder imbalance. In this way, air-fuel ratio imbalances may be accurately identified and mitigated, thereby reducing emissions.

A control device for an internal combustion engine, equipped with: an exhaust purification catalyst capable of storing oxygen; a downstream-side air-fuel ratio sensor arranged downstream in the direction of flow of exhaust from the exhaust purification catalyst; and an air-fuel ratio control device that controls the air-fuel ratio such that air-fuel ratio of the exhaust flowing into the exhaust purification catalyst reaches a target air-fuel ratio. The control device changes the target air-fuel ratio to a lean air-fuel ratio setting when the exhaust air-fuel ratio detected by the downstream-side air-fuel ratio sensor reaches a rich air-fuel ratio, and then changes the target air-fuel ratio to a slightly lean air-fuel ratio setting before the exhaust air-fuel ratio detected by the downstream-side air-fuel ratio sensor reaches a lean air-fuel ratio, and then changes the target air-fuel ratio to a rich air-fuel ratio setting when the exhaust air-fuel ratio detected by the downstream-side air-fuel ratio sensor reaches a lean air-fuel ratio, and then changes the target air-fuel ratio to a slightly rich air-fuel ratio setting before the exhaust air-fuel ratio detected by the downstream-side air-fuel ratio sensor reaches a rich air-fuel ratio.

A method of diagnosing a fault in a common rail fuel system having a proportional-integral-derivative (PID) controller includes determining a first integral output corresponding to a first fuel flow condition and a first rail pressure setting. The method includes comparing the first integral output with a threshold integral output and determining a second integral output corresponding to a second fuel flow condition and the first rail pressure setting, when the first integral output is greater than the threshold integral output. The method includes determining a third integral output corresponding to the first fuel flow condition and a second rail pressure setting, when the first integral output is greater than the threshold integral output. The method includes identifying a failure in at least one of a flow control valve arrangement and a pressure relief valve of the common rail fuel system based on the first, second, and the third integral outputs.