Very low gas milage, engine light coming on and off randomly

So my reliable mechanic of many years died right before I was taking my car in about the engine light. I have to find a new, trustworthy mechanic but I want to know what to expect (as I am not an expert in these things).

In the past, my engine light would come on and go off randomly and my regular mechanic would scan and it would always indicate the catalytic converter temperature was too high, but the actual temperature was not abnormal. He suspected it was a faulty sensor, but that was something we were going to address before he happen to pass away.

That aside, I have had increasingly low milage (15-20mpg), rough idling, and inconsistencies with when the ICE runs. The car has 207k miles, so I anticipate issues. I have had various parts replaced recently, including:

• New hybrid battery
• welds on catalytic converter and exhaust leaks
• Cleaned O2 sensors
• Repaired connector to inverter
• Replaced front struts
• Replaced oil pressure switch
• Replaced 4 spark plugs with Iridium Gen 3
• Replaced radiator, thermostat, water pump
• Replaced mass air flow sensor

I was hoping for some suggestions and info about major causes of the bolded symptoms above. What should I expect when I take it in, and what do you recommend may be the problem?

 

Yes. I recently had it cleaned. I forgot to mention that in my list.

 

Just finished an oil change and there was no evidence of coolant in the oil. Coolant levels are perfect. The engine doesn’t misfire, which I would expect from gasket failure. Any other ideas? I’ve never heard of being able to drive around easily with a blown head.

The occasional rough idle/start is not shaking; it’s almost as if their isn’t enough power. Sometimes the ICE starts to turn off but then doesn’t fully complete and then comes back on.

I would expect the hybrid system indicator to flag if the battery was bad. Plus it’s a new battery.