2000 Pontiac Montana. Error code PO135 O2 Sensor Heater Circuit Malfunction Bank 1 Sensor 1

Here's a link for codes P0446,P0443. https://www.obd-codes.com/p0446 Here's the link for P0403. P0403 Exhaust gas recirculation EGR circuit malfunction Here's code P0135 P0135 OBD II Trouble Code O2 Sensor Heater Circuit Malfunction Bank 1... Here's code P0141 P0141 OBD II Trouble Code O2 Sensor Heater Circuit Malfunction Bank 1... Here's code P0155. P0155 OBD II Trouble Code 02 Sensor Heater Circuit Malfunction Bank 2... Here's the last code.yay P0161. P0161 OBD II Trouble Code HO2S Heater Circuit Malfunction Bank 2 Sensor 2... The last 3 codes might just be one sensor.

P0306 PONTIAC - Cylinder 6 Misfire Detected https://www.autocodes.com/p0306_pontiac.html

P0141 PONTIAC - O2 Sensor Heater Circuit Malfunction Bank 1 Sensor 2
P0141 Pontiac O2 Sensor Heater Circuit Malfunction Bank 1 Sensor 2

The P0135 means O2 Sensor Heater Circuit (Bank 1 Sensor 1)
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wifi obd2 car diagnostic tool

PO135--- Oxygen Sensor Heater Circuit Malfunction
(Bank 1 Sensor 1)

It means you need to replace the upstream o2 sensor. The heater is bad in the sensor.

o2 sensor heater circuit malfunction bank 1 sensor 1 you need to replace the 02 sensor

P0130 - O2 Sensor Circuit Bank 1 Sensor 1
P0135 - O2 Sensor Heater Circuit Bank 1 Sensor 1

Here is a visual aide that will clear up the confusion on where the affected sensor is located.

Given that most of the trouble codes are for the O2 sensor, I think I would change it. The multiple misfires could be because of improper fuel/air mixture caused by a bad sensor. Disconnect the negative(-) battery terminal then change the sensor. Disconnecting the negative(-) should reset the computer and allow the fuel/air mixture to return to default allowing the engine to start.
P0130 - O2 Sensor Circuit Malfunction (Bank 1 Sensor 1)
P0134 - O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 1)
P0135 - O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)
P0141 - O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)
P0300 - Random/Multiple Cylinder Misfire Detected

PO505 is idle control system malfunction. PO446 is evaporative emissions ( evap) vent system performance . PO443 is evaporative emission control sytem purge control circuit malfunction. PO135 is O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1) PO141 is 02 Sensor Heater Circuit Malfunction (bank 1 sensor 2) Now what does all this mean. It means your O2 sensors are being effected by a problem upstream. Could be a many things. If the engine runs perfect then it may be one bad O2 Sensor but it may not. My advice is to have this diagnosed by a trusted mechanic who has the tools, skill, and knowledge to find the problem and not just throw parts at the problem. You can spend lots of money on this unless you know exactly what is causing all these codes.

P0135 02 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)
P0136 02 Sensor Circuit Malfunction (Bank I Sensor 2)
check for a blown fuse, if it is blown replace it, if it blows again after engine is started you have either a shorted O2 sensor or the wiring leading to the up stream or down stream sensors.