Abstract: A method and apparatus for detecting the sufficiency of ink flow through the printhead of an inkjet pen comprises sensing the temperature of the printhead substrate as the inkjet pen prints a test pattern into the printer spittoon. The method compares a first change in temperature resulting from printing one portion of the pattern to a second change in temperature resulting from printing a second portion of the pattern. Based on this comparison, the sufficiency of the ink flow is determined. The comparison is done using a ratio of the second temperature change to the first temperature change. As the ink flow decreases, the second change in temperature rises faster than the first change in temperature and thus the ratio increases. By monitoring the ratio, comparing it to predetermined values and repeating the test pattern on a periodic basis, the method determines when the ink flow has dropped to a point that the pen should be replaced.