I want to start out by saying that I'm not a mechanic and everything in this post was learned over the past few weeks trying to fix this problem. I have a 2001 V6 Camaro (automatic trans, 195,xxx miles). Service Engine Soon light comes on stays on for few days and goes off for a few days repeatedly. I drive 66 miles to and from work so this could be every other drive cycle. I've replaced the MAF sensor, gas cap and both intake manifold gaskets and fixed a couple of vacuum leaks. I don't believe there are any more leaks. I'm getting 22 MPG going back and forth to work and the car is running great. I purchased a scan tool for my laptop... When the light is off. The O2 sensors seem to be working perfectly. B1S1 and B2S1 goes back and forth from about .2V to .8V. The post cat sensor stays at around .75V when idleing. From everything I see on line this is the way they are suppose to look. When the SES light is on the sensors at B1S1 and B2S1 stays near zero at idle and goes up and down between .2V and .8V at about 2000 RPM. The post cat sensor pretty much mirrors the pre cat sensors. It seems to me that the catalytic converter is failing but is it possible for it to work and then stop working then start working again? Again I'm a novice so don't laugh if I make dumb statements. Thanks in advance for any help.

 

It is a lot more likely that the O2 sensor would have an intermittent issue then a Cat would.

The problem is you testing you system based solely on the O2 inputs with no way to verify what the exhaust is. If your primary O2's start misreading the oxygen the PCM will adjust the fuel so it reads what it thinks is right, now the secondary sensor sees the change and it makes it look like the secondary is reading bad.

How old are you Primary O2s? Things like vacuum leaks and coolant getting into a cylinder can be very hard on the O2s. I normally recommend replacement after an intake gasket. I have seen many cars scan almost perfect but after new primary O2 where installed the car got 2-5 MPG more. The scan looks the same it just the new O2s where more accurate.

my 96 averages 30 MPG across a tank in the summer. I drop off to 27-28 in the winter. But I have a stick and 90% highway miles.

 

If you use your scanner, take a look at the fuel trims. Those DTC's are telling you there is too little fuel or too much air. As a result, the PCM will add more fuel by moving up the fuel trim percentages until it can add no more (max limits: +16% for long term and +20% for short term) and that's when the codes set. Since it is on both banks, I would check the fuel pressure next. Most auto stores will loan (deposit required) you a tester. The pressure should read 48-55 psi with ignition on and engine off. With the engine running, it shouldl be 3-10 psi less than without it running

 

Thanks Gary and Gorn. I'll test the fuel pressure and if it's OK replace the o2 sensors. I'll let you know what happens but it will be a few days to get a drive cycle in. I really appreciate you guys taking the time to respond.