Methods and systems are provided for determining a type of spark plug fouling. In one example, a method may include differentiating spark plug fouling due to soot accumulation from spark plug fouling due to fuel additive accumulation based on a current on a control wire of the spark plug following application of a dwell command. Further, exhaust oxygen sensor degradation and/or exhaust catalyst degradation may be determined based on switching frequencies of one or more exhaust oxygen sensors and the type of spark plug fouling.

Methods and systems are provided for determining a type of spark plug fouling. In one example, a method may include differentiating spark plug fouling due to soot accumulation from spark plug fouling due to fuel additive accumulation based on a current on a control wire of the spark plug following application of a dwell command. Further, exhaust oxygen sensor degradation and/or exhaust catalyst degradation may be determined based on switching frequencies of one or more exhaust oxygen sensors and the type of spark plug fouling.