I had my block replaced due to a thrown rod, by SUBARU. 8,000 miles later I now have a cracked head. Subaru scanned my car and revealed that I had a PO171 code, Exhaust running lean, and a PO546 code, Exhaust temperature sensor. I have a cat before the sensor. My question is, can either of these codes lead to a cracked head in any way? My theory is that the exhaust running lean code is because the engine is lean causing extreme temperatures in my engine which explains the exhaust temp code. Extreme temperatures cause cracked head?