P0420 on Alfa Romeo: How to Diagnose & Fix
Catalyst System Efficiency Below Threshold (Bank 1)
1 Alfa Romeo model · 1 documented issue · 1 critical
P0420 on Alfa Romeo by model: symptoms, diagnosis and fix
On 2017-2018 Giulia sedans with the 2.0L turbo four (and 2018 Stelvio), an engine misfire can dump unburned fuel into the exhaust and overheat the catalytic converter. The excess heat can damage nearby engine-compartment wiring and components, which FCA identified as both an engine-compartment fire risk and a stall risk. Owners typically see a check-engine light, rough running and a temporary loss of power, and some report a burning smell, before the stall or fire. This is distinct from the low-pressure fuel pump stall campaign: the root cause here is combustion misfire plus engine-control-module calibration, and the recall centers on updated control software, with inspection and parts replacement if required.
Common Symptoms
- Check engine / malfunction indicator lamp illuminated
- Rough running or shaking at idle
- Temporary loss of engine power
- Burning smell from the engine bay
- Engine stalls without warning
- Smoke or fire from the engine compartment
How to Fix
For an included VIN, recall UA4/18V636 provides a dealer inspection and PCM software update at no charge. The dealer records a scan report and follows FCA's STAR process if P0420, P0171, P2096, P2270 or P0299 is present. The owner notice includes part inspection and replacement when required; consequential damage can require FCA approval through the recall or warranty process. Ask the dealer to determine the covered repair before paying for parts. The estimated visit repair time in the owner notice is about one hour, with additional check-in time.
Call to confirm they can assess your vehicle and perform the stated work. Search results do not confirm qualification, eligibility or coverage.
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 Alfa Romeo?
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 Alfa Romeo it is documented across 1 model: Giulia.
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 Alfa Romeo?
Repair costs vary widely with the root cause and Alfa Romeo model; each issue on this page carries its own estimate where one is published.
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.