Have replaced both Oxy sensors when putting in new catalytic converter. Tried to inspected but trouble code for too rich f/a mixture in a bank. What else could be the problem if those sensors are not

P0420 OBD-II Trouble Code: Catalyst System Efficiency Below Threshold (Bank 1) P0420 code means that either the upstream or downstream oxygen sensor in the catalytic converter is not working. In short your catalyst is worn out.

This means your oxygen sensor is reading problems with your fuel/air mixture. May consider in replacing the oxygen sensor, the one before the catalytic converter.

My definition says bank two sensor one is not switching - stuck on rich.
That means the sensor is telling the computer the fuel mixture is too rich. It could be true and there is a problem with an injector or the pressure regulator, or the sensor could have failed.
Bank two is the side with cylinder number two. Sensor one is before the catalytic converter.

Okay, lets start with you checking the fuel preasure regulator, this is a common problem with these the regulator leaks and the vac line connected to it starts sucking gas into the intake, this causing it to run rich and depending on how bad the leak is it will stall engine at idle, remove the vacuum hose from regulator and start engine then see if gas shoots out the vac connection on the regulator, if it does then replace regulator, hope this helps.

CODE PO420 CATALYST BELOW NORMAL BANK 1 .POSSIBLE CAUSE CATALTIC CONVERTER , WIRING , HO2S 2 HAVE VECHICLE PUT ON A SCANNER TO CHECK FUEL TRIM.A RICH FUEL MIXTURE WILL CAUSE THIS PROBLEM.MAKE SURE SPARK PLUGS AND WIRES GOOD AND FUEL INJCTORS ARE CLEANED NOT CAUSING ENGINE FLOODING MAKE SURE O2 SENSORS OKAY.IF YOU HAVE TO REPLACE THE CATALYTIC CONVERTER. AND FUEL SYSTEM STILL RICH IT WILL BURN OUT NEW CATALYTIC CONVERTER.YOU WILL BE REPLACING IT AGAIN.

Possible O2 sensors:

P2270 O2 Sensor Signal Stuck Lean Bank 1 Sensor 2
P2271 O2 Sensor Signal Stuck Rich Bank 1 Sensor 2
P2272 O2 Sensor Signal Stuck Lean Bank 2 Sensor 2
P2273 O2 Sensor Signal Stuck Rich Bank 2 Sensor 2
P2274 O2 Sensor Signal Stuck Lean Bank 1 Sensor 3
P2275 O2 Sensor Signal Stuck Rich Bank 1 Sensor 3
P2276 O2 Sensor Signal Stuck Lean Bank 2 Sensor 3
P2277 O2 Sensor Signal Stuck Rich Bank 2 Sensor 3
P2278 O2 Sensor Signals Swapped Bank 1 Sensor 3 / Bank 2 Sensor 3
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This is Bank 1 (closer to the dash panel) Sensor 2 (located after the catalytic converter).

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Note: A malfunctioning front oxygen sensor may cause fuel trim codes to set.

1. Several Diagnostic Trouble Codes use specific terminology to refer to sensor location. The illustration and key below explain the following terms:
2. Engine BANK Bank 1 is closer to the dash panel, (also referred to as RH or right hand). Bank 2 is closer to the radiator, (also referred to as LH or left hand).
3. UP Refers to Sensor 1 or "front sensor". It is located closer to the exhaust manifold and before the catalytic converter.
4. DOWN Refers to Sensor 2 or "rear sensor." It is located after the catalytic converter.
5. The following examples show how to interpret oxygen sensor terminology:
6. B1/S1 = Bank 1/Sensor 1 = Dash panel side, BEFORE the catalyst or "upstream"
7. B2/S2 = Bank 2/Sensor 2 = Radiator side, AFTER the catalyst or "downstream"

Change the primary oxygen sensor on the number one cylinder bank. Whichever bank has cylinder number one, is bank one. The primary sensor is the one between the engine and the catalytic converter on that side. There's a secondary sensor behind the catalytic converter, but the primary (pre-cat) sensor is the one used for determining fuel mixture.

When the codes are read they generally have an identifying number for the Oxygen sensors. For instance: "B1S1" This means" exhaust bank one, sensor one. THe first sensor of your first bank.

Your VOLVO 850 is an inline 5-cylinder, so it has just one exhaust bank I believe. So it should have an Oxy sensor upstream and one downstream of the catalytic converter. The upstream one is sensing lean/rich condition of your just-burned fuel mixture, while the downstream one measures efficiency of your catalytic converter. I suggest you go somewhere else to have the car checked and scanned. Nothing against the parts dealers, but those guys are barely cashiers, as you found out. It was free right? Worth just what you paid for it, nothing!!! Maintain your Volvo so it will take care of you for years to come.

P0135--OXY SENSOR HEATER CIRCUIT(bank 1 sensor 1)
P0141--OXY SENSOR HEATER CIRCUIT(bank 1 sensor 2)
P0155--HEATED OXY SENSOR HEATER CIRCUIT(bank 2 sensor 1)
P0161--HEATED OXY SENSOR HEATER CIRCUIT(bank 2 sensor 2)
P01132--LACK OF HO2S11 SWITCH--SENSOR INDICATES RICH
P01152--LACK OF HO2S21 SWITCH--SENSOR INDICATES RICH

Here are the code definitions.hope it helps.

Replace your bank 1 (that's the cylinder bank with cylinder #1 in it, usually passenger side but verify this before changing parts) primary oxygen sensor. This is the one before the catalytic converter. The primary determines your air/fuel mixture, and if it goes bad, the computer will revert to preprogrammed fueling maps, which are fuel-rich by design (to safeguard the engine). Change the O2 sensor on bank one, before the cat, and maybe check your spark plugs as well (running rich too long will foul them) - for the minimal cost, it's worth changing them too.