The treatment status indicator on the TensRelief units flashes red and the treatment has stopped

If the TENS units overheat inside, remove them from the electrodes and let them cool down. As soon as they have cooled down sufficiently, you can reattach them to the electrodes and press the on/off button on the TENS units to resume treatment.
The TENS units may become warm on the outside during treatment, but the temperature never becomes so high that there is any risk of damage or injury.