Ok here's the story.

About a year ago my check engine light started coming on. At first it would just be every now and then occuring spiratically, but it seemed to happen more often on hot days then cold. Now it is to the point where it is on the second the car is started. Of course I ran the code and came back with number 15 the O2 sensor. So after a couple weeks my car had to have some major work done to it that I couldn't do myself so I took it into a trusted shop (Mazda Masters) and asked them to replace the sensor while they were doing all the other work to my car. They replaced it with an OEM sensor from a mazda dealership.

For about two weeks the car was great and the check engine light stayed off. Problem solved right? Nope, after that period the light came on again for the same problem. I didn't have time to take it back to be fixed because I was leaving for college.

Here's where it gets interesting. I bought a Power FC and commander over one of the breaks from school and when I used it there was an interesting problem with the O2. When the car was off and the commander was on the sensor screen it would show the voltage from the O2 fluctuating between (not sure if these are the correct numbers) .01 and .03. Once the car was started the O2 was fine. The commander never highlighted the sensor saying there was a problem. The whole start of the check engine light happened after I installed an intake system on my car. I'm not sure if that was what was responsible, but it never happened before that. I have tried reinstalling the stock airbox with no change.

I've read that this might be a grounding issue, but I'm not certain which is why I'd thought I'd post this to see what you guys think. This wouldn't be too much of a problem but I'm do for CA smog and I won't pass visual inspection with the check engine light on.

I am going to contact the mechanic who worked on it to get some feedback from him.

Thanks alot for your help guys and sorry about the long post.

Jason

 

Stick the stock ecu back in and find out what code it is throwing.

EDIT: Actually, the CEL shouldn't be coming on with the PFC installed...I forget at the moment if the PFC ever turns on the CEL for a function or not.

 

Rynberg,

Stock ECU is in right now and it's still throwing code 15 O2 sensor. As I mentioned this is only a problem with the stock ecu and was not when the PFC was in. I recently sold my PFC so that's why it's become a problem again.

Thanks for your input.
Jason

 

I think the PFC turns the CEL on when the knock is high only. To indicate problems with a sensor it highlights it on the commander. This did not happen to me which is why this problem is different, because the PFC says it's fine while the stock ecu says it's not.

 

Just talked to Ray at Mazda Masters and said he didn't remember seeing anything unusual when he was working on my car. He said it may be another valve/solenoid messing up the air fuel mixture and the computer is thinking it is the O2, but he's not sure.

Anyone else out there got any ideas?

Thanks
Jason

 

Anybody?

 

Replace the O2 sensor if you're that worried about it. It's obviously a sensor issue (they do "go bad" over time).

Or, do what everyone else with a PFC does: options/o2 sensor: OFF.

 

Thanks for the info Clayne, but as I mentioned in my post I did replace the sensor and the problem persisted. I also mentioned that I recently sold the PFC.

 

Well my fault for not reading completely.

There's not really much to the O2 sensor wiring path. BTW: Why did you sell the PFC? The stock ECU is crap in comparison.

 

Well thanks anyways clayne, any input is appreciated.

I sold the Power FC, because I was a little low on cash. Not to mention that my car is going to be sitting in the garage a lot for the next few years as I finish up college and I hate the thought of a $1000+ thing sitting in the garage not getting any use. Better to pass it along and let someone else enjoy it .

 

Well I decided to triple check this morning to make sure I was reading the code that I was getting from the CEL right. Something was just bothering me about the it being the O2 sensor code when the CEL is on from start up. It didn't make sense that the ECU would think the sensor is malfunctioning before the car even warmed up unless the sensor was completely messed up.

I ran the code again, and don't I feel stupid, it is 16 (EGR valve) not 15 (O2 sensor). This makes much more sense as these valves tend to get worn out over time, since my car is a 94 cali it would throw the CEL light like this was the most important part of the car.

Now that I know what the problem is I'm gonna get started on removing and cleaning the valve. Glad that I finally located the problem, and yes from now on I will be triple checking all my engine codes.

Thanks to the forum members who tried to help me troubleshoot.

Jason