P0172 Chevrolet -

Question

first had a PO171 faulty o2 sensor [black smoke, strong foul smell] after sensor was replaced ck light is back on with new code PO172 and 1-2 3-4 shifting condition

Answer 1

Hi there,

P0172 is a rich fuel mixture code. This can be caused by several different items, including a faulty mass air flow sensor.  Leaking fuel injectors or even an issue with the fuel pressure regulator.