P0420 is the catalytic converter inefficiency code. It means your cat isn't cleaning up your exhaust as much as it should be. If your brother had an after-market cat installed in place of the stock cat, that could cause that code to set.

I'm guessing that since the car was pretty well neglected that it's still using the original struts. A new set of struts and springs should make it ride like new again. Other suspension components should be fine unless they were extremely stressed in some way, but I'd still inspect everything and make sure to check all of the bushings in the suspension.
A lot of people run either the kyb gr2 or kyb agx struts, and they seem to be pretty popular. Both ZZP and W-body store carry struts and springs for our cars if you want to take a look at what they have available.

 

P0420 is simply a Cat code. You can take it to a dealer and ask if they will replace it for free under the TSB for it, it not you can always get a ZZP Downpipe and PCM and get the code removed from your PCM haha

 

Welcome to the site!

 

Been a while since I updated...

I've since had the following items fixed -->

rear struts

The return coolant elbow (replaced three times).

Radiator

Right front axle

steering rack

rear exhaust manifold (cracked and leaking)

Car now has over 231K miles. I bought it with approx 206K.

My MIL light is on...and the inspection station failed me for safety and emissions.
The saftey was due to leaking exhaust (now fixed) and no reverse lights.
The emissions portion of the failure is due to several codes - I forget which ones.
Car runs really smooth and strong - the codes really is a buzz kill.
In MA you are not allowed to drive with a rejection sticker for safety - hopefully if I get stopped I can point out that I shelled out $800 for the exhaust leak and the reverse lights are not a big deal - as I am not driving backwards.

PS The wife now is sure I was an idiot for buying this car and continuing to pay for fixing it. I keep figuring its bound to be a new car eventually after I fix every part on it. Problem is I love this car. I will not say how much I have into this thing now.

 

Just joining this forum to comment. I have a 2003 Monte Carlo with almost 273,000 miles. The transmission has been slipping and shaking my car a bit since 260,000. In the winter, the problem is at it's worst. I would have to let the car warm up for 10 minutes for the transmission to operate 80% properly. If I also drive it for more than 2 hours, it will shake every time it switches gears. The body is not in the best shape, so I'm not sure if it's worth fixing. But the motor is insane. I can get from 0-60 in about 4.7 seconds. When the transmission is running fine (a rare thing that happens), the car is really fun to drive. Not sure if spending $800-1000 on the trans is actually worth it.... :/ I love my car but the repairs it needs, makes me feel differently.

 

If your car has A 4.7 0-60 its worth repairing.... is that a guess or actual number... cause these care are 9 second 0-60's stock. Thats IF its an SS

 

Take my advice (or not). Do not fix it - get rid of it. With 273k miles every single component will eventually fail and cause misery not to mention much cash.

I've gone over a year with my 2003 Monte SS with a failed rejection sticker - primarily used to pickup kids from school etc and taking turns any time I see a cop.

I've had the car at the local mechanic several times for the following ODB-II codes -->

P0449 - purge vent solenoid (wire was corroded upstream from rear solenoid) and reverse lights (similar corroded issues/wires pinched) fixed for just under $300.

After driving for 140 miles to reset computer - check engine light - new code.

P0137 - bank 1 sensor 2 - brought it back to mechanic and they replaced the O2 sensor (pre cat location) along with repairing the wiring that led to the o2 sensor. Just under $300 again.

Got car back.
Two new codes (forget which ones) both referring to O2 sensors.

I did my own checking.
P0137 on the GM 3.8L has two heated O2 sensors.
Bank 1 Sensor 1 is pre cat (or upstream)
Bank 1 Sensor 2 is post cat (or downstream)

Notice for P0137 mechanic replaced pre-cat O2 ?
I brought it back and asked them why they repaced sensor 1 when the code referred to sensor 2.
They seemed surprised I was asking...and they said they would look into it.
They did not believe me that sensor 1 is pre cat location and sensor 2 is post cat.
That's okay - I don't believe anyone either...that's why I look up everything and research.
Based on what I can find - P0137 is most assuredly AFTER cat bank 1 sensor 2.
That means they should have performned troubleshooting on after cat O2 NOT pre cat )2.

I will update once I bring the car back.

I bought this '03 SS from my brother for $500 w/206k miles.
I have sunk well over 7k into it in three years.
It has a nice sound system and that was the killer for me.

It's a pity - the motor runs so nice.
Oh - and last year 2013 I also replaced the steering rack which was leaking - I don't think I referenced that expense in this thread. That was over $1000 expense.
Forget the 7k...I'm well over 8k now.
I used to think I was a fairly smart and intelligent fella.
My wife is once again right. I'm an idiot for buying and trying to fix this car.
I hate the thought of a new car payment (or used car payment).
But it is clearly the wiser choice.

Up to now I don't do drugs,pot or gamble.
I may be going down the dark path.
Fixing my car is now an obsession. Must fix. Must drive.

It's like fixing the toilet of a sinking ship. Why bother.

Sorry for the rant....maybe I should steal my brothers '04 jr car haha! Give him back this one.

 

If you fix it yourself its a diferent story.

 

I second dbaldwin. Yea I cut my cost in half with my build by being able to do the work myself.

There not bad cars but they are how old now...... have to expect things to break and if you don't want to repair things that break on a older car then the best solution for you is the new car payment...

 

The previous codes for bad O2 sensors is now gone.
I've also noticed you have to drive to car about 130 miles before the computer is in a 'ready' state.

I have the P420 cat code back - so I'm going to get the cat replaced today.