Free Check Engine Light Diagnosis

I wrote a page about free check engine light services and the problems associated with them. This brought on a question from a visitor that is going down the same road as many before him. You can see the discussion below.


So I just read your article about free check engine light scams. I went to AutoZone they gave me the O2 sensor deal however they did recommend a friendly mechanic instead of selling me stuff.

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The reason I went wasn’t because of the light though it was because my car was stalling out randomly. The mechanic offered me a free diagnosis just because we were from the same area of the country and he related to me.
However he thought the problem was the engine temperature sensor and when he fixed it the problem stopped but then the check engine light came right back on. So I took it back and he said well I guess we can fix the o2 sensor and he did that but the light came back on again.

Now during all this I already knew at the time that my catalytic converter was messed up but the person who inspects my car each year told me that I could get away without fixing it for awhile so I didn’t fix it because before I met the “friendly mechanic” I got a quote somewhere else and the price was ridiculous.

Service engine soon light on


So that leads to today, now I have that problem with the car stalling again coupled with the check engine light.

So I’m wondering if it is time to fix the converter and whether or not putting off fixing it blew out my temperature sensor and o2 sensors again? Any suggestions would be greatly appreciated and thanks a bunch for your time!

Hi Jeff: You did not mention make model or mileage but I wanted to let you know that a catalytic converter if defective is covered under the federal emissions warranty for 8 years 80,000 miles.

If your vehicle is out of this coverage you can compare prices from aftermarket suppliers such as Midas and mienekee. Yes an O2 sensor code can be hard to diagnose and can be caused by a bad converter.

And yes a bad converter can ruin a new O2 sensor. But if I was working on this vehicle I would follow the diagnostic chart for the specific check engine light code that continues to reset.


Hello Mark: It’s a 2000 Toyota Tacoma SR5 TRD Package V6 3.4L Manual Trans. It has 158000 miles and I bought it used in 2003 with 59k mileage on it.

Okay so I have a repair manual, with a diagnostic chart, so if it spits out a code and I follow the chart, how does that work? I saw you said follow the specific code that continues to reset, by reset do you mean the code that keeps causing the engine light to come back on?

Like I said I would like to try and fix it myself, I have a friend that has lots of tools but if it’s something you feel I shouldn’t risk doing on my own please let me know, I would just take it to a shop but I’m just nervous about getting ripped off.

I just moved to California and I am trying to get it registered here but I know I will have to get the engine service light off and the problem fixed in order for it to pass smog inspections.

Plus obviously, I’m having this problem with it stalling again and it does so when I put it in reverse or when I take it out of gear.

And sometimes when I cruise to a stop at a light, basically in stop-n-go traffic, which Cali is full of. Just let me know what you think and thanks again for taking the time out of your day to hear me out!


Hi Jeff: When you clear the codes the one that returns first would be considered the primary code. This would be the one to follow a diagnostic tree chart for.

As far as this being a do it yourself car repair job that would depend on your skill level but also the codes needing diagnosis.

Some check engine light codes are easier to repair than others so it’s hard to say at this point if this will go your way.

In the interest of getting your vehicle fixed in the shortest amount of time it may be wise to have a Toyota dealer run an initial diagnostic and go from there.