Concerning my SS/SC:

So Friday I'm driving.. and the car develops a new noise when coasting sounds like metal against metal rattle. Apply throttle, instant cure.

Saturday.. Rattle is now constant, sounds similar to a loose baffle in the muffler.. Then magically disappears. Moments later, check engine light for a P0420 code. "Catalyst System Efficiency Below Threshold". The car is also now louder in the 3-4k RPM range.

I'm assuming a leak somewhere, would that cause the car to throw this code?

Exhaust is completely stock.

Anybody have a similar issue before, is there a common place for these cars to develop a leak?

 

Is your boost the same?

 

Yeah, it doesn't seem to be affected.

 

FLex pipe cracked ? There is a TSB out that looks like the issue your having .


#08-06-05-002: EI08085 - 2.0L, MIL Lamp On, DTC P0420, Exhaust Noise, Poor Engine Performance (Engineering Information) - (Mar 5, 2008)


Subject: EI08085 -- 2.0L, MIL Lamp On, DTC P0420, Exhaust Noise, Poor Engine Performance (Engineering Information)


Models: 2005-2007 Chevrolet Cobalt SS Supercharged

2004-2007 Saturn ION Redline

with 2.0L Engine (VIN P - RPO LSJ)



Attention: Proceed with this bulletin ONLY if the customer has commented about this concern AND the EI number is listed in GMVIS. If the customer has not commented about this condition or the EI does not show in GMVIS, disregard the bulletin and proceed with diagnostics found in published service information. THIS IS NOT A RECALL -- refer to Corporate Bulletin Number 04-00-89-053B for more details on the use of Engineering Information bulletins.



--------------------------------------------------------------------------------

Condition
Some customers may comment on any of the following conditions:

• Poor engine performance


• Loud exhaust sound


• MIL On (DTC P0420)


Cause
If a customer brings their vehicle in for any of these conditions, complete the following steps before making any repairs:

Using the Tech 2Ž, note any DTCs that have set.



Inspect the exhaust flex joint and note any corrosion, cracks or damage.
Complete the following steps to check the exhaust system back pressure:
3.1. Remove the position 1 HO2S.

3.2. Install the J 35314-A Back Pressure Gage in place of the position 1 HO2S.

3.3. Start the engine.

3.4. Increase and monitor the engine speed at 2,000 RPM.

3.5. Observe and note on the repair order the exhaust system back pressure reading on the gage.

3.6. Turn the engine off and place the ignition in the lock position.

3.7. Remove the J 35314-A and reinstall the position 1 HO2S.

3.8. Remove the position 2 HO2S.

3.9. Install the J 35314-A in place of the position 2 HO2S.

3.10. Start the engine.

3.11. Increase and monitor the engine speed at 2,000 RPM.

3.12. Observe and note on the repair order the exhaust system back pressure reading on the gage.

3.13. Remove the J 35314-A and reinstall the position 2 HO2S.

Clear any DTCs.
Contact the engineers, Brad Clark (248-857-2708) or Larry Szlachta (cell # 248 255 7649), with the results from Steps 1-3.
• If diagnosis finds the flex joint has failed or the converter is plugged, replace the exhaust pipe/catalytic converter. Components will be requested back through the Warranty Parts Center (WPC).

• If diagnosis does not indicate a failed flex joint or plugged converter, continue to diagnose and repair using information found in Service Information (SI).

Warranty Information
For vehicles repaired under warranty and using this bulletin, use:

Labor Operation
Description
Labor Time

L9715*
Exh Pipe/Converter -- Engineering Information
0.8 hr

*This is a unique labor operation for bulletin use only. It will not be published in the Labor Time Guide.

GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They are written to inform these technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions, and know-how to do a job properly and safely. If a condition is described, DO NOT assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See your GM dealer for information on whether your vehicle may benefit from the information.

WE SUPPORT VOLUNTARY TECHNICIAN CERTIFICATION


Š 2008 General Motors Corporation. All rights reserved.

 

Thanks.. I'll check it out

 

Sounds like the cat is self distructing on you.
The peices will clog it soon enough unless it already has.