Abstract: Apparatus and method for combustion analysing a sample comprising determining incomplete combustion in a combustion analyser having a combustion chamber. A sample is supplied to the combustion chamber and combusted to produce combustion products. A target gas characteristic of incomplete combustion of the sample is detected in the combustion products by a target gas sensor. The target gas may be carbon monoxide, methane, methanal and/or methanol, among others. Whether incomplete combustion of the sample has occurred may be determined. A signal indicative of incomplete combustion is output from the target gas sensor and the combustion products can be directed to waste using a valve disposed upstream of the target gas sensor.