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What Is the P0420 Code on a Honda or Acura?
The P0420 code means the ECM has determined that the catalytic converter on Bank 1 is not converting exhaust gases as efficiently as it should. The ECM compares the signal from the upstream (pre-cat) oxygen sensor to the downstream (post-cat) oxygen sensor — when the downstream sensor starts switching rapidly like the upstream one, the cat has lost its ability to store oxygen, which is the core sign of a worn-out catalytic converter.
P0420 is one of the most common codes across the entire Honda and Acura lineup, from the Civic and CR-V up through the Accord, Pilot, Odyssey, and Acura TSX, TL, and MDX. While a worn catalytic converter is the most frequent cause on higher-mileage vehicles, several Honda-specific issues — oil consumption, exhaust leaks at common rust points, and Variable Cylinder Management (VCM) on V6 models — can also trigger it.
Common Symptoms
- Check Engine Light / Service Engine Soon light
- Possible rotten-egg (sulfur) smell from the exhaust
- Failed emissions or OBD2 inspection
- Slightly reduced fuel economy in some cases
- No drivability symptoms in the majority of cases
P0420 Causes on Honda and Acura
| Cause | Likelihood | Models Most Affected | Typical Fix Cost |
|---|---|---|---|
| Worn or failing catalytic converter (age/mileage) | Very High | All models over 100,000–150,000 miles | $400–$1,500 depending on model and OEM vs. aftermarket |
| Faulty downstream (post-cat) O2 sensor | High | All models | $100–$300 |
| Oil consumption from worn valve stem seals contaminating the cat | Moderate | High-mileage K20, K24, and R18 4-cylinder engines | Cat $400–$1,200 plus valve seal job if not addressed |
| Exhaust leak between upstream and downstream O2 sensors | Moderate | All models, especially rust-prone northern climates | $100–$400 |
| Variable Cylinder Management (VCM) misfire or carbon buildup | Moderate | Accord V6, Pilot, Odyssey, Acura TL, Acura MDX (J-series V6) | Varies — often paired with VCM-specific codes |
| Faulty upstream (pre-cat) O2 sensor causing incorrect fuel trim | Lower | All models | $100–$300 |
Model-Specific P0420 Notes
Honda Civic and CR-V (K20, R18, K24 4-Cylinder)
On these four-cylinder models, P0420 most often shows up as the catalytic converter simply ages out somewhere past the 100,000–150,000 mile mark. Before replacing the cat, check for oil consumption — blue exhaust smoke at startup or a burning-oil smell can mean worn valve stem seals are slowly contaminating the converter, and replacing the cat without addressing oil consumption often leads to a repeat failure.
Honda Accord, Acura TSX, and Acura TL (K24 and J-Series V6)
The 4-cylinder K24 versions of these models follow the same pattern as the Civic and CR-V above. On V6 models equipped with Variable Cylinder Management (VCM), check for any stored VCM-related codes (in the P157x range) alongside P0420 — rough transitions between 6-cylinder and 3-cylinder modes can cause minor misfires that, over time, affect downstream O2 sensor readings and cat efficiency.
Honda Pilot, Odyssey, and Acura MDX (J35 V6)
These larger V6 models share the J-series engine family and VCM system. In addition to the oil consumption and VCM considerations above, check the exhaust manifold-to-catalytic-converter joint for leaks — this connection point is a common rust and leak location in vehicles regularly exposed to road salt, and an exhaust leak here can mimic a failed cat on the O2 sensor waveform.
Testing the Catalytic Converter on Honda and Acura Models
O2 Sensor Waveform Comparison
Using a scan tool capable of graphing live data, monitor the upstream and downstream O2 sensors on Bank 1 at the same time, at idle and during a steady cruise. The upstream sensor should switch rapidly between rich and lean. A healthy downstream sensor stays relatively flat; if it starts mirroring the upstream sensor’s switching pattern, the cat has lost its oxygen storage capacity.
Visual and Smell Checks
Check for blue smoke from the exhaust at startup (oil consumption), a rotten-egg smell (cat breaking down), and physically inspect the exhaust manifold-to-cat joint and the area around the cat itself for rust, soot streaks, or signs of an exhaust leak.
Diagnostic Steps
- Read all stored and pending codes — resolve any misfire, fuel trim, VCM, or O2 sensor codes before condemning the catalytic converter
- Check for exhaust leaks between the upstream O2 sensor and the cat, especially at the manifold joint
- Graph upstream vs. downstream O2 sensor waveforms with live data at idle and during a steady cruise
- Inspect for blue or black exhaust smoke that could indicate oil or fuel contamination of the cat
- On V6 models, check for VCM-related codes and inspect for rough cylinder deactivation transitions
- Verify fuel trims are within normal range (roughly +/-5% at idle and cruise)
- If the cat is confirmed degraded, replace with an OEM or CARB-compliant direct-fit unit and address any underlying oil consumption or exhaust leak first
Recommended Scanners for Honda and Acura P0420
P0420 is one of those codes where the right tool can save you from replacing a perfectly good catalytic converter. Look for live data graphing of both O2 sensors and, ideally, bidirectional access to VCM solenoids on V6 models.
- BlueDriver Bluetooth Pro — graphs upstream and downstream O2 sensor data side by side, plus reads VCM-related codes on V6 models.
- Autel AL619 — solid live data graphing alongside ABS access for other warning lights.
- THINKCAR ThinkDiag 2 — its bidirectional functions can actively test O2 sensor heater circuits and, on supported V6 models, VCM solenoids — useful for ruling out a misfire-related cause before replacing the cat.
Related Honda Diagnostic Guides
For our full scanner lineup for Honda-specific codes, see Best OBD2 Scanners for Honda Vehicles. If your Civic or CR-V is also showing an EVAP-related code, see our Honda P1456 EVAP leak guide. A thermostat-related code can also show up around the same mileage range — see our Honda Civic P0128 thermostat guide. For a deeper comparison of the scanners mentioned above, see BlueDriver vs Autel AL619.
Frequently Asked Questions
What does the P0420 code mean on a Honda or Acura?
P0420 means the catalytic converter on Bank 1 is not converting exhaust gases as efficiently as the ECM expects, based on comparing the upstream and downstream oxygen sensor readings. It often points to a worn-out catalytic converter, but exhaust leaks, oil consumption, and O2 sensor faults can also trigger it.
How much does it cost to fix P0420 on a Honda?
A full catalytic converter replacement typically costs $400–$1,500 depending on the model and whether OEM or CARB-compliant aftermarket parts are used. If the cause is a faulty downstream O2 sensor or an exhaust leak instead, the fix is much cheaper at $100–$400.
Is P0420 always caused by a bad catalytic converter on Honda models?
No. A faulty downstream O2 sensor, an exhaust leak near the manifold, or oil consumption from worn valve seals can all trigger P0420 without the cat itself being bad. Always check O2 sensor waveforms and inspect for exhaust leaks and oil consumption before replacing the converter.
Does Variable Cylinder Management (VCM) cause P0420 on V6 Honda models?
VCM itself does not directly cause P0420, but rough transitions between cylinder modes or carbon buildup affecting deactivated cylinders can create minor misfires that, over time, affect downstream O2 sensor readings. If P0420 appears alongside VCM-related codes on an Accord V6, Pilot, Odyssey, or Acura MDX/TL, investigate the VCM system first.
Which Honda and Acura models get P0420 most often?
P0420 shows up across the entire lineup once vehicles pass roughly 100,000–150,000 miles, but it is especially common on the Civic, CR-V, and Accord (4-cylinder), and on V6-equipped Pilot, Odyssey, and Acura TL/MDX/TSX models where oil consumption or VCM-related issues can accelerate cat wear.
