check engine light

[ptns96grandam]

My 96 grand am keeps getting a code po 420 catalyst below threshold. I erase it and it comes back on in about 4 or 5 days. I notice it pops on when im on the beltway going between 55 and 70. I cleared the code friday took to mechanic on monday he didn't find anything even in the soft codes. converter is not broken up intact and doesnt sound cloged he said. I do notice when cold exaust flow is real weak and when it warms up i feel more flow.Car hesitates and idles a little crazy when cold once it warms up its fine. changed plugs last year. Could air control valve map sensor, egr valve be causing code or do you think its converter Car has 150,000 miles

[rixGAphx]

P0420 = Catalyst System Efficiency Below Threshold (Bank 1)

The second O2 sensor (behind the cat) on the '96's only monitors the amount of O2 left in the exhaust after the cat cleans the exhaust.
The PCM compares the value from the second sensor to the primary O2 sensor (in the exhaust manifold).

The PCM then figures whether the cat is doing enough work at burning CO and HC's.

When it sets the P0420 code, the cat isn't working very well.
It *might* be clogged, and cause that code.
But at 150k miles, your cat is not clogged, rather the rare metals in the cat (primarily platinum) are just worn-out, gone.
So the cat isn't effective at cleaning your exhaust, and the PCM is telling you, and the mechanic, that it's time to replace the cat.
This is to be expected, you got more than the 'expected' life out of that cat.

If the second sensor were bad, it would very probably be sending bad information all the time, so the code would nearly always be showing.
The fact that it only throws the code at continuous highway speed indicates it's *prolly* sending accurate readings.

The IAC, MAP, and EGR do cause problems and might *also* be suspect and possibly faulty.
But first, I would just install a new cat, since that is what the PCM says is a problem, it's something known/intended to fail, and it accounts for most of your symptoms.

Good luck.
-Rick

PS: There are good deals thru e-bay and other online sources for mail-order cats. Since this isn't an emergency, you could take a week or two to get one delivered.
Other than dealer mark-up, the main difference in cats is how much precious metal is included in the core. Less platinum = less $$ AND less life. Since you *prolly* only want this car to last another 5 years or so in your hands, there's no need to spend big $$ for a longer-life cat.
Or you could find a freer-flowing cat for less exhaust restriction/greater engine performance.

PPS: When an engine is cold, the exhaust is also somewhat cooler. The 'exhaust flow' out the tips will ALWAYS be less for a given rpm than when that same engine is warmed and the exhaust is hotter. Basic physics, heat = pressure.
So, I wouldn't use that seat-of-the-pants observation as a symptom of the car's present condition.
Exhaust flow may be a factor under other conditions, but I don't see it here.

[ptns96grandam]

Well I broke down and bought converter cost about $200. The converter was cloged and a little melted. Car idles down better whith new converter then before. But still has eratic idle when cold hesitates. I checked plugs look ok. It feels like it has a miss when im driving. Dont know what it could be. Bought some lucas fuel injector cleaner hoping that will help . If anyone has any other ideas let me know. Coils boots and housing replaced about two years ago when I was misfiriring which might have caused converter to go bad