Monte Carlo Forum - Monte Carlo Enthusiast Forums - Repair Guide: 2003 Monte 3.4L: P0404 and P0420

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2003 Monte 3.4L: P0404 and P0420

I'm posting this to help others who may have a similar problem.

Symptoms

First noticed as lack of power while driving in mountains
Later, lack of power when accelerating from 35mph to 55mph on flat highway
Observed that rpms did NOT immediately increase when pressing accelerator pedal
Codes: P0404 and P0420

Action

Cleared codes, waited a week, rechecked: got P0404 only
Researched on internet and concluded problem most likely due to restricted exhaust from bad catalytic converter
Got under vehicle and banged on cat with rubber hammer, but didn't hear anything loose
Found that prices for new OEM cat on internet are around $1,000
Bought replacement cat from Rockauto.com for around $200 (Walker, part #54626)
Bought exhaust manifold to cat gasket and cat to muffler gasket locally along with new cat support rubber rings
Replaced cat (photos below)

Notes

Used 15mm, six-point box combination wrench for rear nuts and 1/2", six-point deep socket for front nuts. DO NOT USE 12 point wrench or socket or you will round off the nut.
The "cheater bar" used was a piece of scrap pipe about 18" long that fit over the wrenches
Could not "torch" the front nuts, but got lucky and they came off without torching.

Nice write up and the pics help as well. Wonder if this pretains to the 3800s as well

I just recently received this code as well. I was wondering if replacing the cat did in fact fix your problem or if something else arose. I just replace the EGR and its still giving me problems.

The only proper repair to the 3.4 is a shotgun. Replaced the top end on my ex-wife's minivan, then about a year later, replaced the entire engine. In other news, the one in my Camaro started spewing white smoke every time it got warm. Thankfully, I traded the car in before it also imploded.

PO300
PO403
PO405
PO440
PO446 ✴︎ ⬅︎ ⬅︎ ⬅︎

All these Codes were triggered by a faulty FUEL TANK PRESSURE SENSOR!

Wait for the Low Fuel Warning before attempting to DIY.
Start engine.
Pull FUEL PUMP Relay ... Let Engine stall to relieve pressure in system.
Remove GROUND Wire from Battery.
With car Raised and SAFETY STANDS securely in place.

Support Tank with Jack.

(I made a "Jig" to attach to the Jack. Sawed the square bottom off from Milk Crate. Drilled holes through Hockey Puck. Mounted Puck to center of Milk Crate with Heavy Duty Zip Ties. Affixed carpet to other side of Milk Crate bottom for padding.)

Lower tank enough to access Sensor atop Fuel Pump.
Unfasten Pigtail from Sensor.
Unclip and Remove Sensor ••• Straight-up.
DO NOT BREAK THE END OF SENSOR OFF INSIDE PUMP!!!

If you need a Charcoal Canister do it when you replace Sensor.

Removing Relay to releive Pressure throws a Code which may require a MIL Light reset.