For what it's worth here is the diagnostic table for a 2000 Century's
temperature control:

Document ID# 610218
2000 Buick Century
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Air Temperature Control Inoperative
Step
Action
Value(s)
Yes
No

1
Was the heater-A/C control Diagnostic System Check performed?
--
Go to Step 2
Go to Diagnostic System Check (See procedure below this one)

2
Inspect the HVAC fuse in the fuse block.

Is the HVAC fuse open?
--
Go to Step 3
Go to Step 4

3
Repair short to ground in circuit 41 (BRN). Refer to Circuit Testing and
Wiring Repairs in Wiring Systems.

Is the repair complete?
--
Go to Step 13
--

4
Turn the ignition switch off.
Disconnect the left electric actuator electrical connector.
Turn the ignition switch on.
Use a digital multimeter (DMM) to measure the voltage between the left
electric actuator harness connector terminal 5 and ground.
Is the measured voltage within the specified range?
9-14V
Go to Step 6
Go to Step 5

5
Repair the poor connection or open in circuit 41 (BRN). Refer to Testing
for Intermittent and Poor Connections and Circuit Testing in Wiring Systems.

Is the repair complete?
--
Go to Step 13
--

6
Use a DMM in order to measure the voltage between the left electric
actuator harness connector terminal 6 and ground.
Vary the driver temperature knob from full cold to full hot.
Does the voltage vary within the specified range?
0.5-4.5V
Go to Step 7
Go to Step 8

7
Replace the left electric actuator. Refer to Temperature Actuator
Replacement (LH) .

Is the repair complete?
--
Go to Step 13
--

8
Inspect for a short to ground or short to B+ in circuit 1199 (DK BLU).
Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Is there a short to ground or short to B+ in circuit 1199 (DK BLU)?
--
Go to Step 9
Go to Step 10

9
Locate and repair the short to ground or the short to B+ in circuit 1199
(DK BLU).

Is the repair complete?
--
Go to Step 13
--

10
Inspect for a poor connection or open in circuit 1199 (DK BLU) between the
heater-A/C control harness connector terminal C7 and the left electric
actuator harness connector terminal 6.

Is there continuity in circuit 1199 (DK BLU)?
--
Go to Step 11
Go to Step 12

11
Repair the poor connection or open in circuit 1199 (DK BLU) between the
heater-A/C control harness connector terminal C7 and the left electric
actuator harness connector terminal 6. Refer to Testing for Intermittent and
Poor Connections and Circuit Testing in Wiring Systems.

Is the repair complete?
--
Go to Step 13
--

12
Replace the heater-A/C control. Refer to Control Assembly Replacement .

Is the repair complete?
--
Go to Step 13
--

13
Verify that the system operates properly.

Does the system operate properly?
--
System OK
Go to Diagnostic System Check

Document ID# 599174
2000 Buick Century
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Diagnostic System Check
Test Description
The number(s) below refer to the step number(s) on the diagnostic table.

Lack of communication may be due to a partial or total malfunction of the
class 2 serial data circuit. The specified procedure will determine the
particular condition.

The presence of DTCs which begin with "U" indicate some of the modules are
not communicating.

Step
Action
Yes
No

1
Install a scan tool.

Does the scan tool power up?
Go to Step 2
Go to Scan Tool Does Not Power Up in Data Link Communications

2
Turn ON the ignition, with the engine OFF.
Attempt to establish communication with the BCM and PCM control module(s).
Does the scan tool communicate with the control module(s)?
Go to Step 3
Go to Scan Tool Does Not Communicate with Class 2 Device in Data Link
Communications

3
Select the control module display DTCs function on the scan tool.

Does the scan tool display any DTCs?
Go to Step 4
Go to Symptoms

4
Does the scan tool display any DTCs which begin with a "U"?
Go to Diagnostic Trouble Code (DTC) List/Type in Data Link Communications
Go to Step 5

5
Does the scan tool display DTC B1000?
Go to Diagnostic Trouble Code (DTC) List/Type in Body Control System
Go to Step 6

6
Does the scan tool display the PCM DTCs P0530 or P1546?
Go to Diagnostic Trouble Code (DTC) List/Type in Engine Controls - 3.1L
engine or Diagnostic Trouble Code (DTC) List/Type in Engine Controls - 3.8L
Go to DTC Table