Trouble Code P0140

I have a 2002 Grand Am with a 3.4. I replaced both O2 sensors, the catalytic convertor and nothing. I bought an O2 simulator and plugged it into the rear and reset the computer. I was good for about 2 weeks and now same light, same code. I'm guessing something maybe wrong with the computer... Where is the computer located on these cars?

I think I would look at the data for the rear O2 sensor with a scan tool and with a good voltmeter at the sensor. Id there is no data on the scan tool but data on the volt meter than there is a bad wire/connection somewhere.

First, check fuse #47 (Canister Vent Valve, Exhaust Oxygen Sensors) in the hood fusebox.

I would also look at the PCM side, the connection could be oxidated.

Obiviously, check for wire damage.


From: http://www.obd-codes.com/p0140

P0140 - 02 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)

OBD-II Trouble Code Technical Description

02 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)
What does that mean?

The Powertrain control module (PCM) will provide a .45 volt reference voltage to the Oxygen sensor. When the O2 sensor reaches operating temperature, it will generate a voltage that will vary depending on the oxygen content of the exhaust. Lean exhaust generates a low voltage (less than .45V) and rich exhaust generates a high voltage (greater than .45V). O2 sensors on a specific bank marked as "sensor 2" (as this one is) are used to monitor emissions. A Three-Way Catalyst (TWC) system (catalytic converter) is used to control tailpipe emissions. The PCM uses the signal received from Oxygen sensor 2 (#2 indicates aft of catalytic converter, #1 indicates pre-converter) to read efficiency of TWC. Normally this sensor will switch between high and low voltage at a noticeably slower rate than the front sensor. This is normal. If the signal received from rear (#2) O2 sensor indicates that the voltage has "stuck" between .425V to .474 V, the PCM determines this sensor is inactive and this code will set.

Fuse #47 is fine. Can anyone tell me where the pcm is located on this car? Is it in the passenger side kick panel?

The PCM is under left side of instrument pannel, near the steering column.

I'ld check for the wires condition and connector pins for oxydation.

Pinout for the Heated O2 sensors:

Sensor 2: Cat. out
Brown: 12V in run position, from fuse 47
Black: ground
Tan/white: PCM pin 28: H2OS 2 low
Purple/white: PCM pin 11: HO2 sensor 2

Sensor 1: Right exaust manifold
Brown: 12V in run position, from fuse 47
Tan: intake air temperature sensor (at left of engine)
Purple: PCM pin 10: HO2 sensor 1 (heated o2 sensor)

IAT:
Tan: to O2 sensor 1
Black: to PCM pin 12: Sensors ground

Update: When the muffler shop installed the cat last year they didn't weld the pipe all the way. I found a 1/4 gap on the top of the pipe before the cat. I'm almost certain that was the cause of the P0140 code. Thanks for all your guy's help.