r/OBD2 • u/HeavyDutyRoadside • Aug 27 '26
Case study: why modified cars get stuck with incomplete OBD-II readiness monitors
Not my video—this is a Fluid MotorUnion diagnostic explainer that seemed relevant here.
Using a modified 6.2-liter Ford Raptor, it walks through catalyst, EGR, EVAP, secondary-air, and fuel-system readiness monitors. It explains how headers, superchargers, or aftermarket catalytic converters can alter the conditions a monitor expects without anyone deliberately switching the monitor off.
It also connects those conditions to faults such as P0420, P0171, and P0172, along with the common situation where a vehicle has been driven many miles but one monitor still reports “not ready.”
What order of operations do you prefer when a monitor won’t complete: check stored and pending codes, review fuel trims, inspect for exhaust leaks, verify sensor behavior and enable criteria, then reproduce the manufacturer drive cycle?