P0420: Catalyst system efficiency below threshold, bank 1
What the code tells you on its own
- System
- Powertrain
- Functional group
- Catalytic converter
- Subsystem
- Auxiliary emission controls
- Failure type byte, when the tool sends one
- 00 (No sub-type information)
Engine, fuel, air, ignition, emissions and transmission. The overwhelming majority of what workshops bring to the AI.
Scan tools report the same fault as P0420 and as P0420xx, where xx is a sub-type. They are one fault, and whether the sub-type appears at all depends on the tool and the protocol rather than on the car.
Diagnostics
What we see when P0420 turns up
Measured across the diagnostics run through Wrenchlane, not taken from a reference table. Figures as of 2026-10-06.
- Diagnostics
- 35
- Separate workshops
- 24
- Arrived code only
- 40%
- Most common marque
- Chevrolet
April 2026 to September 2026
Not one shop meeting it repeatedly
No symptom description entered at all
Where we have seen it most
These are counts from our own work, not a claim about how common the fault is across all vehicles. They describe what independent workshops brought to us, which is a narrower and more specific thing.
Real cases
Real cases with P0420
Diagnoses of P0420 on real cars, written up: the car, the codes it came in with, the causes ranked and what to check first.
P0420
What P0420 usually turns up with
Measured across our own diagnostics. The multiplier is how much more often the two appear together than chance would predict, so a high number means the pair is telling you something a single code is not. Codes rarely arrive alone, and reading the set is usually faster than chasing the first one on the list.
| Code | What it is | Seen together | Versus chance |
|---|---|---|---|
| P1133 | Manufacturer-specific, meaning depends on the marque | 3 times | 54.9x |
| C0561 | Not individually documented | 4 times | 36.6x |
| C0899 | Not individually documented | 4 times | 36.6x |
| P0751 | Shift solenoid A performance or stuck off | 4 times | 36.6x |
| P0700 | Transmission control system, MIL request | 4 times | 31.4x |
| P0300 | Random or multiple cylinder misfire detected | 3 times | 3.7x |
FAQ
Frequently asked questions
How do I run a diagnosis?
Select the vehicle on Start, click the Diagnostic card and choose New diagnostic. Fill in the description, the error codes and the mileage, then click Analyze.
Do I have to type the codes from my scan tool?
No. Click the camera button next to Error codes and take or choose a photo of the scan tool's screen; WrenchLane reads the codes and their descriptions into the form. You can also speak them with the microphone.
Are the public fault code pages built from my diagnostics?
They use diagnostics run in WrenchLane, with no workshop, customer, registration number or VIN shown. Each fault code page shows how often the code was seen and in how many workshops.
Run P0420 against the car in front of you
Enter the code and the symptoms and get ranked probable causes for that specific vehicle, with the checks in the order worth doing them. Free account, no card.