The effect of combustion chamber deposit (CCD) thickness, in specific vehicle engines, upon tailpipe emissions over the US FTP cycle was investigated. The CCD measurement technique takes piston centre CCD thickness measurements through the spark plug access hole and does not require an engine teardown. This allowed the change in emissions with varying mileage to be directly correlated to the CCD thickness. A non-linear dependence upon CCD thickness was observed for NOx emissions during CCD build-up from clean engine conditions and subsequent aging over extended mileage. Furthermore, removal of up to 50% of CCD thickness gave no observed change in emissions in “unaged” (ca. 6000 mile) CCD's but gave a NOx increase to peak emissions in aged (extended mileage) CCD's.