What is happening with my car when the engine light comes on with the three codes, P0171, P0174, P0402? Can I still run it safely until I fix it
; and what exactly needs to be fixed?
My car has an automatic transmission.
P0171 and P0174 signify lean combustion, that is there is too little fuel in the air-fuel mix that is being combusted in the cylinders. Lean operation can occur due to an intake vacuum leak but in your circumstance with the third code, namely P0402 (EGR excess flow), what you might have is an EGR system malfunction that leads to the lean operation. Ford models use a differential pressure sensor in the EGR system. The sensor responds to EGR flow when the EGR valve is open. If that sensor fails it can cause the 2 lean codes that you are reporting to set. That is just one possibility; obviously there are others, too, including that the lean operation is caused by some other fault other than an EGR fault and the cause of the EGR system code (P0402) is due to some cause other than a faulty sensor. Lean operation can cause overheating and so while this is not an emergency the malfunction should be pinpointed and resolved as soon as practicable. If you want the required diagnostic steps performed by a certified Mechanic, dispatched by YourMechanic right to your location, please request a check engine light diagnostic - lean operation and the responding certified mechanic will get this resolved for you promptly. 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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