P0420 on Mitsubishi: How to Diagnose & Fix
Catalyst System Efficiency Below Threshold (Bank 1)
1 Mitsubishi model · 1 documented issue · $300-$2,500 repair
P0420 on Mitsubishi by model: symptoms, diagnosis and fix
The 3.0L 6G72 and 3.8L 6G75 V6 engines in the Eclipse GT develop excessive oil consumption over time. Worn valve stem seals and piston ring glazing allow oil to enter the combustion chambers, producing blue exhaust smoke and requiring frequent top-offs.
Common Symptoms
- Blue smoke from exhaust
- Oil level dropping between changes
- Fouled spark plugs
- Catalytic converter degradation
How to Fix
Replace valve stem seals as a first step. If consumption persists, the piston rings may need replacement. Use high-mileage synthetic oil with seal conditioners. Monitor oil level every 1,000 miles.
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 Mitsubishi?
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 Mitsubishi it is documented across 1 model: Eclipse.
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 Mitsubishi?
Published repair estimates on Mitsubishi range from $300 to $2,500 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.