Heater works when engine is running at high rmp's but cold otherwise

I have a 2004 Hyundai Elantra. The heater will put hot air out at high RPM, but cold air at low RPM. The check engine light came on, had a code of 128. I replaced the thermostat and engine coolant sensor, had the light turned off. 4 days later, the engine light is back on with a 128 code. Heater, same condition as above. Do you have any suggestions as to what the cause is?Thank you in advance.