Is this a 4 or 6 cylinder machine?

It appears to me that the powertrain control module controls both cooling
fans based on what the single engine coolant temperature sensor is telling
it.

Here's the diagnostic procedure for P0128:

Document ID# 715430
2001 Oldsmobile Alero
--------------------------------------------------------------------------------
DTC P0128
Circuit Description
An engine coolant temperature (ECT) sensor monitors the temperature of the
coolant. This input is used by the powertrain control module (PCM) for
engine control and as an enabling criteria for some diagnostics.

The air flow coming into the engine is accumulated and used to determine if
the engine has been driven within conditions that would allow the engine
coolant to heat up normally to the thermostat regulating temperature. If the
coolant temperature does not increase normally, or does not reach the
regulating temperature of the thermostat, the diagnostics that use ECT as
enabling criteria may not run when expected.

This DTC will only run once per ignition cycle within the enabling
condition. If the PCM detects the calibrated amount of air flow and engine
run time have been met, and the ECT has not met the minimum thermostat
regulating temperature, DTC P0128 sets.

Conditions for Running the DTC
The engine is running.
DTCs P0112, P0113, P0117, P0118 are not set.
The ECT is more than -40°C (-40°F).
The ECT is less than 78°C (172°F).
The intake air temperature (IAT) is more than -7°C (+19°F).
The engine has been running for more than 2 minutes, but less than 30
minutes.
The vehicle speed is more than 24 km/h (15 mph) for more than 1.609 km (1
mile).
The mass air flow (MAF) average reading is more than 15 g/s.
Conditions for Setting the DTC
The PCM detects the following:

The calibrated amount of engine run time has been met.
The calibrated amount of engine air flow has been met.
The calibrated ECT of 80°C (176°F) has not been met.
Action Taken When the DTC Sets
The control module illuminates the malfunction indicator lamp (MIL) on the
second consecutive ignition cycle that the diagnostic runs and fails.
The control module records the operating conditions at the time the
diagnostic fails. The first time the diagnostic fails, the control module
stores this information in the Failure Records. If the diagnostic reports a
failure on the second consecutive ignition cycle, the control module records
the operating conditions at the time of the failure. The control module
writes the operating conditions to the Freeze Frame and updates the Failure
Records.
Conditions for Clearing the MIL/DTC
The control module turns OFF the malfunction indicator lamp (MIL) after 3
consecutive ignition cycles that the diagnostic runs and does not fail.
A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
A history DTC clears after 40 consecutive warm-up cycles, if no failures are
reported by this or any other emission related diagnostic.
Clear the MIL and the DTC with a scan tool.
Step
Action
Values
Yes
No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine Controls Connector End Views or
Powertrain Control Module (PCM) Connector End Views

1
Did you perform the Diagnostic System Check-Engine Controls?
--
Go to Step 2
Go to Diagnostic System Check - Engine Controls

2

Important
The cooling fans are commanded ON when certain engine coolant temperature
(ECT) DTCs are set.

Is the cooling system coolant low?
--
Go to Draining and Filling Cooling System in Engine Cooling
Go to Step 3

3
Test and verify the proper operation of the thermostat. Refer to Thermostat
Diagnosis in Engine Cooling.

Did you find and correct the condition?
--
Go to Step 14
Go to Step 4

4
Disconnect the ECT sensor.
Inspect for the following conditions:
Corrosion on the ECT sensor terminals
Improper or corroded terminals at the ECT harness connector
Loose terminals in the ECT harness connector. Refer to Testing for
Intermittent and Poor Connections and Connector Repairs in Wiring Systems.
Did you find and correct the condition?
--
Go to Step 14
Go to Step 5

5
Measure the voltage from the signal circuit of the ECT sensor to a good
ground with a DMM. Refer to Circuit Testing in Wiring Systems.

Is the voltage within the specified range?
4.8-5.2 V
Go to Step 6
Go to Step 8

6
Measure the voltage from the signal circuit of the ECT sensor to the low
reference circuit of the ECT sensor with a DMM. Refer to Circuit Testing in
Wiring Systems.

Is the voltage within the specified range?
4.8-5.2 V
Go to Step 9
Go to Step 7

7
Test the ECT sensor low reference circuit for high resistance. Refer to
Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?
--
Go to Step 14
Go to Step 11

8
Test the ECT sensor signal circuit for high resistance. Refer to Circuit
Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?
--
Go to Step 14
Go to Step 11

9
Turn OFF the ignition.
Remove the ECT sensor. Refer to Engine Coolant Temperature (ECT) Sensor
Replacement .
Place the sensor on a work surface away from any heat source.
Allow the sensor to reach the ambient air temperature for 30-60 minutes.
Observe and record the ambient air temperature of the vehicle environment
using an accurate thermometer.
Measure the resistance of the ECT sensor and record the value.
Compare the resistance measurement of the ECT sensor to the ambient air
temperature on the Temperature vs. Resistance table. Refer to Temperature vs
Resistance .
Is the resistance measurement of the ECT sensor within the specified range?
--
Go to Step 10
Go to Step 12

10
Install the ECT sensor. Refer to Engine Coolant Temperature (ECT) Sensor
Replacement .

Is the action complete?
--
Go to Intermittent Conditions
--

11
Test for an intermittent and for a poor connection at the PCM. Refer to
Testing for Intermittent and Poor Connections and Connector Repairs in
Wiring Systems.

Did you find and correct the condition?
--
Go to Step 14
Go to Step 13

12
Replace the ECT sensor. Refer to Engine Coolant Temperature (ECT) Sensor
Replacement .

Did you complete the replacement?
--
Go to Step 14
--

13
Replace the PCM. Refer to Powertrain Control Module (PCM) Replacement .

Did you complete the replacement?
--
Go to Step 14
--

14
Clear the DTCs with a scan tool.
Turn OFF the ignition for 30 seconds.
Start the engine.
Operate the vehicle within the Conditions for Running the DTC. You may also
operate the vehicle within the conditions that you observed from the Freeze
Frame/Failure Records.
Did the DTC fail this ignition?
--
Go to Step 2
Go to Step 15

15
Observe the Capture Info. with a scan tool.

Are there any DTCs that have not been diagnosed?
--
Go to Diagnostic Trouble Code (DTC) List
System OK