I can smell some exhaust under the hood. Engine is running smoother than usual. Anyone have experience with this?

P0 170/173 are most likely for the SAME problem(part). Most common failure is the mass air sensor.

The P0 400 is not as common but normally means that the EGR valve is defective OR that the EGR tube is plugged with carbon...OR BOTH.

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I'm going to attempt to clean the MAF Sensor and maybe remove some carbon from the EGR valve --- I'll borrow a Craccord from a buddy if I break it too much and it won't run :-) Wish me luck!

Smell of exhaust is probably not relevant to codes. Re the 400 code,You definitely have to pull the EGR tube to clean the end of the pipe. It unscrews from the EGR valve. You will have to pull the entire intake ductwork and the rubber vacuum distribution block to get clearance for this pipe to come out of the manifold. It WILL BE clogged at the end.