check engine light has come on twice, took to dealer for service

Was driving car and engine light came on, took to dealership, and was told I needed a thermostat. Then 3 weeks later, engine light came on again, and took back to dealership, and was told I needed a water pump, had same thing happening both times, so not sure needed a water heater and thermostat both, if a diagnostic was run then why didn't this show up the first time car was taken in.