Mechanic replaced knock sensors, wireharness, coolant hose,new gaskets, plugs, PCV, Thermostat. Now getting codes174 and 171 triggered at highway speed. Car runs good. Some loping at idle. No obvious vacumn leaks. 2003 Avalon XLS with 150K miles. Wondering if using propane to detect leak is still appropriate-feel this could be a little dangerous. Any help would be appreciated.