Here it is out of the book:
Probably a vacuum leak, or if not then look for a faulty MAF sensor.

Mike

Code 2-3-1 (Long Term Fuel Trim Partial Load) Or
Code 2-3-2 (Long Term Fuel Trim Idling)

1) Check for other codes. If Code 3-1-5 is stored, perform
testing on that code first. If Code 3-1-5 is not stored, go to next
step.

2) Check status message on Volvo Scan Tool (998-8686). If
status message reads LOWER LIMIT, go to step 8). If status message
reads UPPER LIMIT, go to next step.

3) Turn ignition on. Connect voltmeter between MAF sensor
connector terminals No. 2 and 4. If voltmeter reads .1-.2 volt, go to
next step. If voltmeter does not read .1-.2 volt, test using new MAF
sensor.

4) Check for air leakage in intake system. Repair as
necessary. If air leakage in intake system is not present, go to next
step.

5) Check for air leakage in exhaust system. If there is no
air leakage, go to next step. If there is air leakage, repair as
necessary.

6) Check fuel and residual pressure. If fuel and residual
pressure are okay, go to next step. If fuel and residual pressure are
not okay, repair as necessary.

7) Connect breakout box to ECM and check ground connections.
See COMPUTERIZED ENGINE CONTROLS in I - SYSTEM/COMPONENT TESTS
article. Reconnect ECM. Repair as necessary.

8) If status message reads LOWER LIMIT, turn ignition on.
Connect voltmeter between MAF sensor connectors terminals No. 2 and 4.
If voltmeter reads .1-.2 volt, go to next step. If voltmeter does not
read .1-.2 volt, test using new MAF sensor.

9) Check for air leakage in intake and exhaust system. Repair
as necessary. Check fuel and residual pressure. Repair as necessary.
If no air leakage and fuel and residual pressure are okay, go to next
step.

10) Remove hose leading to intake manifold from EVAP valve.
Connect vacuum pump to EVAP valve coupling. If vacuum gauge does not
show stable vacuum, repair as necessary. If vacuum gauge does show
stable vacuum, go to next step. Check engine oil and fill as
necessary.

11) Start engine and run at idle a few minutes. Using Volvo
Scan Tool (998-8686), read off value on SHORT TERM FUEL TRIM and note
value. Disconnect and plug crankcase ventilation. Read off value of
short term fuel trim once again. If value is more or less unchanged,
go to next step. If value rises, engine oil is diluted with fuel.
Repair as necessary.

12) Connect breakout box to ECM connector and check grounds.
Connect ECM. Check all signals to and from ECM. Repair as necessary.