Saturn and other diagnostic experts, help needed.

Co-worker has been troubleshooting this over the cube wall with me for months and I don't really know what else to tell him. He's not a car guy but at least he's an OCD computer geek so his data collection has been good.

He has a 97 Saturn SL2. He has owned it for about a year an a half now. In that entire time approximately every 300 miles it throws code P0420 Catalyst System Efficiency Below Threshold.

In 2011 the car had a new exhaust system including a catalytic converter and lower O2 sensor installed. Less than 30k on the car since then. Paperwork to prove all that. No paperwork for an upper 02 sensor so that is my first guess.

He went to the trouble of getting diagnostic software and has been mapping the ODB2 readings and both O2 sensors are putting out what appears to be appropriate readings.

The normal things have been done, full tune up, coolant sensor, etc.

He has also noted that it only appears to happen after he has had the car out on the highway, above 50mph for 5 minutes or more. Since it's his "around town" car this is something he is able to note with some consistency since it doesn't get above 50mph often.

Other than telling him to replace the upper O2 sensor since it's age can't be verified any other thoughts?

He has checked the entire system for leaks and has not found any.

Oem or aftermarket exhaust? If aftermarket, what brand?

Oem or aftermarket exhaust? If aftermarket, what brand?

Depending on what killed his OE cat, I could see an aftermarket one dying in 30k miles.

How's his oil consumption?

No oil consumption issues. No other issues which would cause the premature death of a cat.

I believe it is an aftermarket setup, unfortunately from a small shop out of state so no warranty there.

i had a bad COP coil on my Mazda6s, ran it about 40 miles on 5 cylinders, still injecting but not sparking on the sixth. after i replaced the coil, it started setting P0420. there's a narrow set of conditions in which that code will set, hence your coworker's experience with setting on the highway but not around town.

google "P0420 conditions" or something similar, maybe even thrown in the year / make / model to the search and you may find how to make it go away. i think the pre-cat O2 is a good bet.

Has the ECTS ever been replaced?

Could be causing the problem directly, could be causing the problem that killed the cat too. They can go bad without ever throwing any codes, and make the car do some really weird E36 M3.

ProDarwin wrote: Has the ECTS ever been replaced? Could be causing the problem directly, could be causing the problem that killed the cat too. They can go bad without ever throwing any codes, and make the car do some really weird E36 M3.

That was what I was thinking. I remember changing one of those babies out. Stupid plastic tip. I forget how you get one with the brass tip from autozone since that POS was like 4 cars ago now.

And also, I call BS on the no oil usage. Saturns never die, people kill them, check your damn oil.

long history of low quality gas?

O2 Sensor? was that replaced with the new exhaust?

Edit: nevermind, I see it has new O2 sensors.

In reply to EricM:

I think it only has a (verified) new downstream sensor.

From http://www.obd-codes.com/p0420:

The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar.

I have it in my head that a relatively unresponsive (old) O2 sensor may not react quickly enough to give the relative variations the system is looking for.

I'd say I went through this with my van, but I really did have a dead cat in the end...