Q: EGR Valve or Mass Air Flow Sensor: Which is more likely for the poor startup and rough idling that I have been experiencing?

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A year ago, I started my car up and it shook violently. I shut it down and tried again several times before finally deciding to just put it into drive and see what happened. It moved for a few seconds and then the entire car power shut off. After waiting 10 min, it started up fine and I didn't run into a similar problem for the whole year again till last week.

This week, the car cranked but did not start until an hour later after it cooled down. Recently, the car would struggle to start, and it had rough idling. However, when the car was in motion, it was totally fine (no misfires, ran excellent on the highway).

The check engine light came on this week and gave me P0170 and P0171 codes. Autozone suggested an EGR valve replacement based on this. However, I've also read that the MAF could be responsible for my symptoms as well.

Based on the symptoms I have provided, is it possible to make a guess as to whether the EGR valve or MAF sensor is more likely to be a fault?

My car has 192272 miles.
My car has an automatic transmission.

The codes do not necessarily implicate the EGR system or the MAF sensor as a multitude of other faults are as likely. Basically, the codes suggest there is too little fuel, or too much air, in the cylinder charge (the A/F ratio) on Bank 1 cylinders. The cause behind diagnostic trouble code P0171 is easiest to find by looking at the secondary firing patterns on the three Bank one cylinders. That will confirm if the too lean condition, that has caused the code to set, affects all cylinders in the bank. If not all cylinders are affected, test for defective individual fuel injectors. However, if all cylinders are affected, you would test, in turn: the fuel pressure regulator, fuel pump pressure, check for a clogged fuel filter, vacuum leak, faulty oxygen sensor(s) and faulty mass air flow sensor. Rarely, the cause will be a faulty powertrain control module. If you want these steps performed by a certified Mechanic, dispatched by YourMechanic right to your location, please request a check engine light diagnostic - code P0171 and the responding certified mechanic will get this taken care of for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.

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