Renew CCI plan and complete heating pm check. Find lennox 11 year old furnace operating normal, find flame safety weak and worn. Find furnace producing high CO level of 68 PPM. Suggest combustion calibration to help reduce CO level and improve efficiency. Suggest replacing flame safety to prevent failure. Find gas line and flue pipe not up to code. Customer do not want repair at this time. Heating system operating at this time. Change filter customer provided.