P0420 on Cadillac: How to Diagnose & Fix
Catalyst System Efficiency Below Threshold (Bank 1)
1 Cadillac model · 1 documented issue · $900-$2,000 repair
P0420 on Cadillac by model: symptoms, diagnosis and fix
On 2013-2017 XTS with the 3.6L LFX V6, the weld at the catalytic converter mounting flange cracks, producing an exhaust leak at the front of the engine. Owners first notice a ticking or hissing that sounds like an intake or valvetrain noise on cold start, followed by an exhaust smell inside the cabin and eventually P0420/P0430 catalyst-efficiency codes as the leak upsets the downstream oxygen sensor reading. GM documented the condition first in preliminary information PIP5419 and then in TSB 17-NA-321 (November 2017), which calls for converter replacement rather than weld repair. The bank 2 (front) converter is the usual failure point.
Common Symptoms
- Ticking or hissing exhaust noise on cold start that fades as the engine warms
- Exhaust or fume smell inside the cabin, especially at idle or with the HVAC on fresh air
- Check engine light with catalyst efficiency codes
- Visible soot at the converter flange
How to Fix
Inspect the converter-to-pipe flange welds with the engine cold and running; soot tracks or an audible leak at the flange confirm it. Replace the cracked converter assembly with the updated GM part per TSB 17-NA-321 (bank 2 converter GM 84216836 is the commonly cited updated part), then clear codes and verify the catalyst monitor completes. Welding the crack is a temporary repair because the stainless flange re-cracks. There is no recall; GM covered replacements under the 8-year/80,000-mile federal emissions warranty for the converter when the car qualified, so check the in-service date and mileage before paying out of pocket. Out of warranty, parts are roughly $800-$1,400 plus about 2-3 hours of labor.
Sources for this issue (3)
P0420 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0420 mean on a Cadillac?
P0420 stands for "Catalyst System Efficiency Below Threshold (Bank 1)." P0420 means the engine computer determined the catalytic converter on Bank 1 isn't cleaning the exhaust as efficiently as it should. The computer compares the upstream and downstream oxygen sensors; a healthy converter stores and releases oxygen, so the downstream sensor should read fairly steady, while a worn one lets its readings mirror the upstream sensor. When that efficiency falls below a set threshold, this code is set. While it most often points to a worn-out catalytic converter, it can also be triggered by upstream problems like a faulty oxygen sensor or an exhaust leak, so the converter isn't always the actual fault. On Cadillac it is documented across 1 model: XTS.
What are the most common causes of P0420?
The most common causes of P0420 are: Worn-out or failing catalytic converter (Bank 1), Faulty or aged downstream (or upstream) oxygen sensor, Exhaust leak before or near the oxygen sensors, Engine running rich or lean (fuel trim issues), Misfires or oil/coolant contaminating the converter, Damaged or aftermarket low-quality catalytic converter, Faulty oxygen sensor wiring or connectors, Software/calibration needing update (on some vehicles). The specific cause varies by vehicle.
How much does it cost to fix P0420 on a Cadillac?
Published repair estimates on Cadillac range from $900 to $2,000 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
Looking for P0420 on a different make?
View P0420 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.