Most modern ethylene cracking furnaces use close-coupled extractive or in situ oxygen analyzers based on a zirconium oxide sensor in the stack area to provide fast and reliable measurement of the oxygen content of the flue gas exiting the firebox. This allows for precise control of the excess combustion air in the firebox, resulting in higher energy efficiency for the furnace than is typical using conventional extractive sample-based analyzer systems where the analyzer might be several hundred feet away from the sample point. These analyzers use a zirconium oxide sensing element that operates at 700 degrees C, which is well above the auto-ignition temperature of many flammable hydrocarbon mixtures. Although flame arrestors can be installed on these analyzers to prevent flame propagation from the probe element into the firebox there are still situations that can exist that the analyzer can pose as an ignition source. This paper will discuss the two typical zirconium oxide oxygen analyzer designs and how the analyzer can be a source of ignition, and review an incident in a Dow cracking furnace, which resulted from an in situ oxygen analyzer igniting a flammable mixture in a furnace. Corrective actions are included in this paper.