P0420 on Cadillac
Catalyst System Efficiency Below Threshold (Bank 1)
P0420 on Cadillac vehicles indicates catalyst system efficiency below threshold (bank 1). Au7o has documented this code across 1 Cadillac model — most commonly on XTS. 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. Typical repair costs on Cadillac range from $900 to $2,000, depending on the specific model and root cause.
Common Causes of P0420
- •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)
P0420 on Cadillac by Model
Cadillac XTS(1 issue)
- Catalytic Converter Flange Weld Cracks Causing Exhaust Leak and Cabin Fume Smell (TSB 17-NA-321)2013-2017
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.
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View P0420 across all makes →Frequently Asked Questions
What does P0420 mean on 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 specifically, this code is documented across 1 model.
What causes P0420 on Cadillac vehicles?▼
Common causes on Cadillac: 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. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P0420 on a Cadillac?▼
Repair costs on Cadillac range from $900 to $2,000, depending on the specific model and root cause.
Which Cadillac models have P0420 documented?▼
Au7o has documented P0420 on 1 Cadillac model: XTS.