Volvo: check engine light, fuel air mixture, mass air flow

Question
I have 1996 850 turbo wagon check engine light is on
put snap on scanner on and found code po172
could you please tell me wich o2 sensor to replace
before cat or after cat
thank you

Answer
Jag,
Code PO172 is Volvo 231/232
2-3-1 LONG TERM FUEL TRIM PARTIAL LOAD

Precondition for DTC
The control module receives information from the Heated Oxygen Sensor (HO2S ) about the fuel/air mixture in the idling and partial load ranges. If the fuel/air mixture deviates from Lambda=1 the short term fuel trim will compensate this by adjusting the injection timing so that Lambda=1 is achieved. When the short term fuel trim carries out this adjustment, its mid-point must be adapted by the long term trim. When the long term fuel trim has adapted the short term trim to a maximum value, DTC 2-3-2 (idling range -0.48 ms to +0.40 ms) or DTC 2-3-1 (partial load range 0.773 to 1.227) and status message Upper Limit (lean fuel/air mixture) or Lower Limit (rich fuel/air mixture) will be set.

Substitute values
None.

Fault sources
Upper limit:
Defective Mass Air Flow (MAF ) sensor.
Low fuel pressure.
Air leakage.
Faulty sensor signals.
Lower limit:
Defective Mass Air Flow (MAF ) sensor.
Air leakage.
High fuel pressure.
Leaking injectors.
Leaking EVAP system.
Dirty engine oil.
Oil level too high.
Faulty sensor signals.

Fault symptoms
Can result in high fuel consumption.
Can result in impaired performance.