2000 Nissan Pathfinder Diagnostic Code 217?

Hello and thank you for any help!

Regarding a Nissan Pathfinder. Heater wasn't heating that well, fiance noticed a "syrupy" smell. Clock was dimming, her clicker/transmitter wasn't working. Finally, check engine came on for 2 days, then clicked off by itself for day, then back on.

Took it in to a shop. Battery is dead so we will replace that. We'll replace the battery for the transmitter as well. They did a pressure test as well. They found the coolant was low. As if the engine had overheated. She had never noticed the gauges do so, or the car respond in any way with smoke, etc. They suggested a coolant flush/power steering flush cause they were both "dirty". I have read about them, some say good and necessary, some know. The power steering. Is it worth it if she's never noticed anything? Or will it prolong the life of her car (which would be good).

They checked for leaks regarding the coolant, couldn't find any. Told her the diagnostic code for the check engine was 217 (someone else I spoke to said it could be PO217?) Having to do with the inlet/outlet thermostat that needed replacing, as the car hadn't responded to the overheating with the gauges. A couple of shops I asked about the code 217 said they'd never heard it, nor had Nissan. Nissan suggested driving the car more with the new battery to see if the check engine came back on. Cost for doing the thermostat is $400.

Is it worth it?

Thanks again for any help!

Camilo