The fuel injection equipment (FIE) has always been paramount to the performance of the Diesel engine. Increasingly stringent emissions regulations have dictated that the FIE becomes more precise and sophisticated. The latest generation FIE is therefore less tolerant to deposit formation than its less finely engineered predecessors. However, the latest emissions regulations make it increasingly difficult for engine manufacturers to comply without the use of exhaust aftertreatment. This aftertreatment often relies on catalytic processes that can be impaired by non-CHON (carbon, hydrogen, oxygen and nitrogen) components within the fuel. Fuel producers have therefore also been obliged to make major changes to try and ensure that with the latest technology engines and aftertreatment systems the fuel is still fit for purpose. However, there has recently been a significant increase in the incidence of reported problems due to deposit build-up within vehicle fuel systems. Understanding the underlying processes leading to this problem is complicated by the coincident change in vehicle specification to meet the latest emissions limits, the change in fuel sulphur specification and the increasing use of bio-diesel. Various analytical techniques have been used to study these deposits in order to prove or disprove different hypotheses regarding the formation mechanism leading to these deposits.Temperature programmed oxidation (TPO) is an analytical technique which can be used to determine the oxidation reactivity of the carbonaceous deposits. This may yield additional information regarding the structure of the carbonaceous deposits now being encountered. This paper describes the technique as it has been applied in analysing deposits from vehicle fuel filters and high pressure fuel injectors. The results from the application of TPO to a variety of the currently encountered deposits is presented and placed within the context of other analytical work used to determine possible causes for this current spate of field problems.