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LS1 · 1998 · COOLING / FAN CONTROL

1998 LS1 Cooling-Fan Diagnosis

Diagnose 1998 LS1 fan complaints by separating PCM command, relay logic, fan power/ground, motor operation and actual cooling-system performance.

APPLICATION: 1998 Firebird / Formula / Trans Am 5.7L LS1 (VIN G); exact-year 1998 cooling/fan-control architecture.
SYMPTOMMake the failure repeatable.
TESTMeasure the system in the failed state.
ISOLATESeparate input, control, output and mechanical branches.
PROVEDemonstrate the failed branch before buying parts.
REPAIRRepair the proven cause.
VERIFYRepeat the same test state after repair.

Name the failure precisely

Neither fan, one fan, wrong operating stage, constant operation and overheating despite running fans require different branches.

Prove the input state

Confirm coolant-temperature data and operating conditions are plausible before deciding the PCM should or should not command the fans.

Command to output

Follow the chain from PCM fan request to relay action to motor power/ground and fan movement. Do not skip directly from a hot gauge to a motor.

Mechanical cooling branch

If electrical fan operation is proven, diagnose coolant level/pressure, airflow restriction, thermostat/flow and engine heat generation rather than staying in the electrical tree.

Modified-car checkpoint

Fan-on calibration changes, lower-temp thermostats and aftermarket wiring can intentionally change behavior. Compare against the current tune and wiring, not only factory expectations.

DO NOT REPLACE YET

Do not replace relays, fan motors, ECT sensor or PCM until command and output have been separated.

FPV DIAGNOSTIC RULE
SYMPTOM → TEST → ISOLATE → PROVE → PARTS VERDICT → REPAIR → VERIFY. A DTC, gauge reading or symptom chooses the next test; it does not choose the part.
1998 starting/charging →1998 diagnostic architecture →Exact year navigator →Start over by symptom →
PUBLISHABLE SOURCE

Exact-year verification trail

This page intentionally omits hard values that are not reproduced from an exact-year source on the page. Use the linked exact-year service information for connector-level tests, pinouts and numerical limits.

AFTER THE FAILURE IS PROVEN

Use the proven failed branch—not the original symptom—to choose the replacement. Go from diagnosis to the Parts Verdict →