Fault code P0300 and P0131

Hello everyone,

I have a 2006 Mazda5 110k miles and it now gave me two fault code

1. P0300: Random Multiple Cylinder Misfired detected
2. P0131: O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1)

I have a couple questions to ask cause I google the last couple days and now don't know what to do.
1. How to test the ignition coil using ohm meter?
2. Is the misfired cause the O2 sensor defect or the O2 sensor defected cause the misfired?
3. Where is the O2 sensor (Bank 1 Sensor 1)
4. Is it true if the EGR dirty cause the misfired also?
5. What I need to fix first? The misfired or the O2 sensor? And what and where to start first?

Thank you and appreciate for all the idea and help.
T

P0131: The front heated oxygen sensor (or O2 sensor 1) is placed into the exhaust manifold.

Possible causes:

- Faulty Front Heated Oxygen Sensor Bank 1
- Front Heated Oxygen Sensor Bank 1 harness is open or shorted
- Front Heated Oxygen Sensor Bank 1 circuit poor electrical connection
- Inappropriate fuel pressure
- Faulty fuel injectors
- Intake air leaks may be faulty
- Exhaust gas leaks

Replace the sensor first, reset codes and see what happens.
The sensors goes bad with age and since it tells computer about proper mixture that needs to be injected, I'd start with that sensor and see where it goes after.
It is a cheap enough item to try.

P0300
Possible causes:


- Faulty spark plug (s)
- Faulty ignition coil (s)
- Clogged or faulty fuel injector (s)
- Intake air leak
- Fuel injectors harness is open or shorted
- Fuel Injectors circuit poor electrical connection
- Ignition coils harness is open or shorted
- Ignition coils circuit poor electrical connection
- Insufficient cylinders compression
- Incorrect fuel pressure

I'd start with replacing spark plugs, by your mileage it is time.
You can have coils tested by a shop or try at home. This is a video but not for mazda.

I'd also perhaps clean the throttle body, and run a fuel injector cleaner. I had good luck with: https://www.amazon.com/dp/ (commissions earned)

Be careful with that sensor. You would want to tighten it slightly to break the seal before you loosen it. I sheared off the threads when replacing mine

Swenny said:

Be careful with that sensor. You would want to tighten it slightly to break the seal before you loosen it. I sheared off the threads when replacing mine

Click to expand...

So true. It is best to get the car warm enough so the threads are not as tight, but not so hot you burn your hands. Also make sure to use gloves.
Depending on the rust, I'd also try to spray penetrating oil a night before the change, just make sure to clean it will after you get the sensor out as you don't want to contaminate the new one.

As you can see, common item is air leaks. so check that all the clamps on the intake are tight and no perforations. Also clean MAF sensor with MAF sensor cleaner (not brake cleaner) and check your air filter.

Thank you all for your input.

I just order spark plug, coil and o2 sensor.
I look into the throttle body and it really dirty inside. I would like to take the throttle and EGR out to clean them both. Is that necessary?

I also want to buy the hose that run from the throttle to the EGR in case it all clogged up inside. What does that hoes name?
Thank you,
T