I am trying to fix my daughter's Blazer. It starts and gives a P0135 code and also a P0300 code. Should I replace the 02 sensor? Are there 4 of them?

 

P0135 - HO2S Heater Circuit Bank 1 Sensor 1
P030x - Random/Multiple Cylinder Misfire Detected

I would diagnose the P0135 code first as it may be the cause of the P0300 code.

The number of O2 sensors depends on the year of the truck. B1S1 is the driver side upstream O2 sensor which is in the y-pipe just below where it connects to the driver side exhaust manifold.

 

It is a 2001 Blazer, VIN 1GNCT18W81K165391

The code I et says P0135 02 sensor heater circut MF Bank1 sensor 1

 

You only have 3 O2 sensors, but the specific code is for the B1S1 (bank 1 sensor 1) which I had already stated.

The links I posted above will explain the codes and how to troubleshoot them.

You could try swapping the passenger side upstream sensor (closest to the passenger side exhaust manifold in the y-pipe) with the driver side O2 sensor and see if the code changes to P0155. If it does, the sensor is likely at fault. If it doesn't, then there is a wiring problem causing the issue.

One last note, use AC Delco or NGK O2 sensors if you do end up replacing them!

 

Thanks much. I will attack it tomorrow. Will this condition cause the engine to run rough and sputter, balk when gas pedal is pushed?

 

Possibly, yes.

 

I had a P0135 code yesterday for the bank 1 O2 sensor and also a P0300 random multiple cylinder misfire detected code. I pulled the O2 sensor and of course the sensor was extremely black so I installed a new one. Started righ up but it balks at taking gas when the gas pedal is pushed. I let it warm up and it balks when accelerator is pushed. I am now getting a P0106 code, Map/Baro circut range/perf and also the P0300 code again. It will barely start now.

I also changed the distributor cap and rotor; the vehicle has143,613 miles on it and the cap and rotor were probably the originals.

 

I'm going to merge this back into your other thread as the problems are very likely related.

Here's the diagnostic sheet for your current code:
P0106 - Manifold Absolute Pressure [MAP] System Performance

 

Check fuel pressure and leakdown, this thread will help you: https://blazerforum.com/forum/showthread.php?t=45941