OBD code information sheet
https://dvireports.com/p0420/audi
Inefficient Catalytic Converter(s), Defective Front or Rear Oxygen Sensor(s), Misfiring engines
Selected definitionAudi1 definitions availableChange
Choose the make reported by the scan tool or vehicle documentation. The same code can have different manufacturer meanings.
Technical explanation
Inefficient Catalytic Converter(s), Defective Front or Rear Oxygen Sensor(s), Misfiring engines. Source catalogue definition; confirm applicability using vehicle service information.
Check the fuses before ordering a part
An OBD code describes a detected condition; it does not tell you which part to replace. Start with the fuse for the affected circuit.
If the fuse is good, carry on like this:
- Check the relevant supplies and connectionsWhere the diagnostic procedure calls for electrical checks, verify supply, ground and connectors under the manufacturer’s activation conditions.
- Run the specified component testUse the manufacturer’s test for this code and component before deciding it has failed.
- Order only after the fault is demonstratedReplace a part only when the prescribed tests identify it as defective.
Check details
- Identify the circuit and any relevant fuse: Confirm the vehicle, engine, code meaning and stored operating conditions first. A fuse link is a location aid, not evidence that the fuse caused this code. Use the specified type and rating; do not repeatedly replace a fuse that opens.
- Check the relevant supplies and connections: Use the specified engine state, circuit conditions and expected values. Have live measurements performed by someone qualified. For non-electrical faults, follow the relevant mechanical, fluid, pressure or leak checks instead.
- Run the specified component test: Do not apply a generic resistance range. Any resistance or continuity test requires a de-energized circuit and an isolated component, with the correct pins and temperature from the procedure. Some components need a different kind of test.
- Order only after the fault is demonstrated: If results are normal, continue the relevant wiring, control, mechanical or intermittent diagnosis. Check for related faults and verify the repair under the required conditions. A code or a completed checklist alone does not justify replacement.
If the procedure involves airbag/SRS, pretensioners or a high-voltage system, stop DIY testing and use qualified service. Do not probe these circuits with a multimeter.
Vehicle experiences and cases
User-reported cases published after moderation. Approval does not verify the diagnosis; the same code can have different causes.
No experiences have been published for this definition yet. You can be the first to share the checks and outcome.
See all vehicle experiencesManuals, components and vehicle guides
Find the relevant procedure in the workshop manual for your vehicle. A fault code alone does not identify a failed part, powertrain or exact schematic.
Manuals open at their original source. Exact diagram availability depends on the build year, engine and manufacturer access.
How to fix P0420: step-by-step guide
Symptoms, causes, visual inspection, scan-tool or multimeter tests, replacement and clearing the code, with safety warnings.
Oxygen sensing and heating
General system guidance · not a confirmed diagnosis
Measurement and heater circuits are distinct; interpretation depends on sensor technology.
The association is based on the imported description. This information explains the system; it does not confirm a procedure for the exact code, model or engine. Verify applicability in manufacturer documentation.
Manufacturer-specific definition: verify its meaning and subtype with a compatible tool before using this general guide.
Checks for this type of system
- Identify the bank and sensor number.
- Have the specified circuit and exhaust integrity checked.
Useful diagnostic data, where available
- Sensor voltage/current appropriate to the technology.
- Heater command where available.
Normal values and thresholds vary by vehicle and are not assumed here.
Mistakes to avoid
- Do not read a wideband sensor as a narrowband sensor.
- A heater fault does not prove a failed catalyst.
System-guide sources
References support general principles, not individual verification of every associated code.
Vehicle associations in the catalogue
Catalogue associations help searching and do not confirm code applicability for every model, year and engine combination. Check the vehicle service information.
Make
Continue your research
Links to related systems, warning lights and vehicle examples.
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