I have a 2010 Terrain with the 3.0 direct injection. Keep getting codes P0172 and P0175 system too rich codes for both banks. I have also had code for engine misfire on cylinder 4 and just recently on cylinder 1. A week ago I had a mechanic run full diagnostics and stated it was an injector problem. They did an upper cleaning and lower fuel injector cleaning and changed the spark plugs which ended up costing $500.00. One week to the day the check engine light came back on. It does not seem to be running bad and the gas mileage is around 24 mpg combination. The mechanics stated that if the check engine light came back on it could mean replacing the injectors which means pulling the heads and that cost is around $3000.00. The vehicle has 123500 miles on it. I have cleaned the mass air flow sensor and the throttle body. Is there anyway of bench testing the mass air flow sensor to see if it is bad? I am open to suggestions please.