Vehicle: 2008 Ford F-350 Super
Duty, 2WD, V10-6.8L, Automatic Transmission
Problem: The truck hesitates and bogs on
acceleration. Malfunction Indicator Light is on.
Details: : The tech pulled Diagnostic Trouble
Codes P0306 and P0420. Checking PIDs with a scan tool, he found
that the Fuel Trim adjustments (long- and short-term fuel trims
combined) were Bank 1 = -7% and Bank 2 = +22%. He conducted an
exhaust back pressure test on both banks and found high back
pressure on Bank 1.
Confirmed Repair: Based on his findings, the
tech concluded that since this truck had a converter on each
bank, when one bank was restricted, no fresh air from intake went
into that bank, causing the fuel trims to go negative. All the
fresh air went into the other bank, causing the mixture to go
lean and fuel trims to go positive. They tech found the catalytic
converter was plugged. He replaced it and cleared the DTCs. The
truck now runs normally.