A diagnostic method, comprising: i) providing at least a signal comparator threshold value, a positive signal amplifier saturation threshold value, a negative signal amplifier saturation threshold value less than said positive signal amplifier saturation threshold value, ii) providing a current value of a first amplified input signal at a first time and a current value of a second amplified input signal obtained at a second time later than said first time, iii) comparing a difference between said current value of said first amplified input signal and said current value of said second amplified input signal to said signal comparator threshold value and providing a new output state based on the comparison, iv) providing a current output state from a previous comparison, and determining an expected new output state based on said current output state, v) if said current output state is equal to a first value and said new output state is different from said expected new output state, checking whether said current value of said second amplified input signal exceeds said positive signal amplifier saturation threshold value and/or whether said current value of said first amplified input signal is below said negative signal amplifier saturation threshold value, and if so, providing a diagnostic signal that indicates a fault, or: if said current output state is equal to a second value and said new output state is different from said expected new output state, checking whether said current value of said first amplified input signal exceeds said positive signal amplifier saturation threshold value and/or whether said current value of said second amplified input signal is below said negative signal amplifier saturation threshold value, and if so, providing a diagnostic signal that indicates a fault. i) providing at least a signal comparator threshold value, a positive signal amplifier saturation threshold value, a negative signal amplifier saturation threshold value less than said positive signal amplifier saturation threshold value, ii) providing a current value of a first amplified input signal at a first time and a current value of a second amplified input signal obtained at a second time later than said first time, iii) comparing a difference between said current value of said first amplified input signal and said current value of said second amplified input signal to said signal comparator threshold value and providing a new output state based on the comparison, iv) providing a current output state from a previous comparison, and determining an expected new output state based on said current output state, v) if said current output state is equal to a first value and said new output state is different from said expected new output state, checking whether said current value of said second amplified input signal exceeds said positive signal amplifier saturation threshold value and/or whether said current value of said first amplified input signal is below said negative signal amplifier saturation threshold value, and if so, providing a diagnostic signal that indicates a fault, or: if said current output state is equal to a second value and said new output state is different from said expected new output state, checking whether said current value of said first amplified input signal exceeds said positive signal amplifier saturation threshold value and/or whether said current value of said second amplified input signal is below said negative signal amplifier saturation threshold value, and if so, providing a diagnostic signal that indicates a fault.