cold idle lasts for a long time, currently has DTC P2178 (too rich off idle)

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2015 Mazda 5 Sport
Hello,

My 2010 Mazda 5 Sport has the following symptoms in a northeast of the US area where the temperature right now is about 0-30F.

The P2178 (too rich off idle) code showed up two weeks ago. The oxygen sensor has been replaced with the Mazda stock part by the garage. They could not find the cause of the issue.

I noticed that when I start the car in the morning and let it idle, even after 10 minutes the coolant temp. gauge is not moving above the cold line. But once I start driving it, the coolant temp. indicator would go to the normal (little below half) after less than a minute of driving.

I can smell sweet smell when the car is in idle in cold. The oil does not look milky. So I assume it's the unburnt fuel the computer gives the engine during open loop. The fuel economy is about 17mpg mixed driving while it's rated between 22 and 28mpg.

Right now I am thinking about the clogged PCV valve, purge solenoid valve, or faulty MAF ground. I am not sure if faulty injector or fuel pump (some recalls on those for Mazda 3 in earlier years) should be tested. Any suggestions are appreciated. I checked with Autozone and their code scanner does not seem to support freeze frame. Though I may just go there and have them to confirm if there are any pending codes other than the P2178 I got from Torque app (with ELM237 OBDII dongle).
 
Sweet smell you would think be coolant leaking at/near the heater core. Coolant leaks and the subsequent air pockets will cause erratic idle. I would refill/burp the cooling system and then check for leaks. Perfect tool for that would be a pressure test kit.
 
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