Monitoring circuitry for an implantable stimulator device is disclosed. A switching matrix allows current from a current source to be distributed to any of a plurality of electrodes. A voltage drop across the active switches in the switch matrix is monitored and is compared to an expected voltage based upon the amplitude of the current and the known on resistance of the switch. If the monitored and expected voltages differ significantly, then a failure condition can be inferred, and an appropriate action can be taken, such shutting down stimulation. Using the already-existing switches in the switching matrix in this fashion is beneficial because it allows the current through the electrodes to be monitored without providing additional structures in the therapeutic current path, which would increase complexity and add unwanted resistance.