The piezoresistive effect of thick-film resistors has found widespread application in pressure and force sensors due to their high strain-sensitivity, stability and low-cost. The main material requirements for these applications are a high piezoresistive response, low noise and low temperature dependence. However, many useful elastic substrate materials are degraded at the standard thick-film processing temperature of 850 °C, and therefore require a lower one. This is achieved by engineering the glass composition. Here we discuss how the composition and microstructure affect the electronic properties of thick-film resistors with both high and low processing temperatures. Depending on the firing temperature, we find percolative or non-percolative behaviors in the range of Ru02 concentrations studied. We show that for percolative samples a lowering of the Ru02 concentration generally leads to a logarithmic increase of the piezoresistive response, which poses serious limitations regarding the associated power- law increase of noise.