Is spray dye something I can get at say, O'Riellys auto parts or someplace like that? I've never had to buy that before.

 

How's your gas mileage?

I know they sell dye in any parts store over by the Freon, it's used a lot to pinpoint leaks in the cooling system.

 

Dye penetrant for detecting surface breaking cracks is not same as AC dye.

I looked and don't think parts stores carry this product. It was available at Grangers Supply but cost $19 for a spray can.

good luck

 

It's something a machine shop would use (not an AC repair shop), so maybe someplace that sells supplies like that.

 

Not sure about the AF sensor but O2 sensors depend on external oxygen reference to measure O2 content in the exhaust stream. I have read of O2 sensor errors caused by exhaust leaks near the O2 sensor. If the crack is real, this may be source of false P0420.

I suggest using a dremel tool to grind and polish the potential crack. Then spray w/ dye penetrant. If crack appears, it's real.

Note, some cracks don't open and allow gas to move through. Have you attempted to observe the crack w/ engine fully warmed ?

good luck

 

Yes, the AF sensor needs an outside reference also. It's really just an O2 sensor that has enough of a proportional signal, not just on/off.

Nothing like that can measure absolute O2 (I hope the EPA never requires anything like an onboard GC).

 

I had a similar problem with a Toyota and aftermarket Cats. The car still ran fine after changing O2 sensors, cat and having the exhaust checked for leaks. Some aftermarket Cats are junk.

What I finally did was install a spark plug anti-fouler in the after cat O2 sensor position and then installed the O2 sensor. That will put the light out and all is well. That was about 30,000 miles ago,.