1998 LS1 Diagnostic Architecture
1998 already has a broad OBD-II diagnostic set. Use the exact 1998 chart rather than borrowing a later-year threshold or pinout.
Air metering
P0101/P0102/P0103 MAF families are present. Treat performance, low-frequency and high-frequency faults as different diagnostic problems.
Oxygen-sensor families
Upstream/downstream signal and heater DTC families are separate. A heater code is not the same diagnosis as a mixture-response code.
Fuel trim
Both-bank lean/rich families are available. Diagnose shared causes when both banks move together before condemning two sensors or two banks of injectors.
Ignition / misfire
Individual coil-circuit DTCs and misfire monitoring are part of the 1998 LS1 strategy. Coil-near-plug architecture changes the fault tree from LT1 Optispark cars.
Catalyst and EVAP
P0420/P0430 catalyst-efficiency and multiple EVAP leak/control DTCs are present. Downstream O2 activity is evidence in a catalyst test, not an automatic sensor verdict.
EXACT-YEAR GATE
Do not copy a 2001/2002 enable criterion, voltage threshold, connector pin or calibration assumption into a 1998 diagnosis unless the exact-year material proves it matches.