Got a 2003 CTS and I just had the thermostat and housing replaced to fix the P0128 code. Heat before wasn't that great, but now the heat on the passenger side is decent but the heat on the driver side is almost nil. Temp is set at 90 for both sides. Is this likely a cooling system problem, or is this more likely an electronic problem? And so far the code hasn't come back. Thanks in advance for the help.