A microelectromechanical ink jet printhead includes a number of groups of ink ejection nozzles. Each group has a temperature sensor that is monitored by a processor incorporating the printhead. In order to maintain printing ink at a constant viscosity the processor compares temperature signals from the sensors to a predetermined threshold. In the event that the temperature signals indicate that the printhead is below a predetermined temperature then the processor applies heating pulses to the printhead. The heating pulses are of sufficient energy to cause warming of the printhead ink but insufficient to cause actual ink ejection. In this way ink ejection droplet size, and hence print quality, is maintained in spite of operating temperature fluctuations.