Honda-specific code — factory diagnostic data| DTC Data Sheet | |
| Code | P0420 |
| Vehicle | Honda Accord (2003-2024) |
| Engine | 2.4L K24 / 3.5L J35 V6 / 1.5L L15 Turbo / 2.0L K20 Turbo |
| System | EMISSION SYSTEM |
| Fault type | Performance |
| Official meaning | Catalyst System Efficiency Below Threshold (Bank 1) |
Definition source: Honda factory description. Diagnostic guidance is based on factory-defined fault logic for this code.
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Looking for the cross-vehicle definition? Read the generic P0420 article for the SAE-defined fault logic that applies to all manufacturers.
P0420 Quick Answer
P0420 on a Honda Accord is unusually often a genuine catalyst issue past 130k miles — the K24 and J35 catalysts have lower precious-metal loading than truck cats, so they genuinely wear out. Before condemning the cat: check the rear O2 sensor (Honda spec is ~100k), the thermostat on the 1.5L turbo (TSB 19-103), and the J35 V6’s well-known exhaust manifold cracks (2008-2012).
What Does P0420 Mean on a Honda Accord?
The Honda Accord (2003-2024) stores P0420 when the condition described above is met. This guide covers all Accord engine variants from the 2.4L K24 4-cylinder to the 3.5L J35 V6 and 1.5L L15 turbo. Model-specific failure patterns documented below differ from the generic SAE definition.
Symptoms
- Check Engine Light (P0420)
- No driveability symptoms
- Slight fuel economy drop
- Possible sulfur smell under load
- On 1.5L turbo: long warm-up time = thermostat
- OBD-II / smog inspection failure
Common Causes (Most Likely on This Model First)
- Worn catalyst substrate. The K24 4-cyl and J35 V6 cats genuinely wear out around 130-150k miles. Honda cats are precious-metal-light for cost-effective emissions compliance.
- Rear (Bank 1 Sensor 2) O2 sensor lazy. Honda O2 sensors good for 100-120k miles. After that, slow switching false-triggers P0420.
- Cracked exhaust manifold (2008-2012 V6 J35). Honda TSB 11-035 covers exhaust manifold cracks on 8th-gen Accord V6 models. Causes false-lean reading.
- 1.5L turbo thermostat (2018+ TSB 19-103). Stuck-open thermostat keeps engine below 180°F. Catalyst monitor fails.
- Oil dilution on 2018+ 1.5L turbo. TSB 17-091 affected Accord too. Oil mist contaminates the catalyst.
Diagnostic Approach
- On 1.5L turbo: check coolant temp at idle — under 180°F sustained = TSB 19-103 thermostat.
- Live-data B1S1 and B1S2 at warm idle. S1 switching 0.1-0.9V; S2 flat ~0.7V. Mirroring = cat failed.
- B1S2 switching frequency at steady 40 MPH > 0.5/sec = cat done.
- On 2008-2012 V6: inspect exhaust manifold for cracks per TSB 11-035.
- On 1.5L turbo: check dipstick for fuel smell, elevated level (oil dilution TSB 17-091).
- Past 130k miles with everything else clean, cat is likely cause. Use Honda OE or DENSO direct-fit.
Possible Fixes
| Fix | When |
|---|---|
| Replace rear O2 sensor | Sensor > 100k miles, slow B1S2 |
| Replace thermostat (1.5L turbo) | Coolant temp < 180°F sustained |
| Replace exhaust manifold (V6 2008-2012) | Crack visible per TSB 11-035 |
| Replace catalytic converter | Honda OE / DENSO direct-fit only — universal cats fail again |
| Oil dilution TSB service (1.5L turbo) | Fuel-diluted oil |
Can I Still Drive With P0420?
Yes — no driveability impact. Drive indefinitely from a mechanical standpoint. Smog-inspection states will fail it.
How Serious Is This Code?
Low. The 1.5L turbo thermostat case is worth fixing quickly since prolonged cold-running has long-term wear costs.
Repair Costs
| Repair | Cost |
|---|---|
| Rear O2 sensor (Denso OE) | $140 – $240 |
| Thermostat (1.5L turbo) | $220 – $380 |
| Exhaust manifold (V6, TSB) | $680 – $1,200 |
| Catalytic converter (Honda OE / DENSO) | $720 – $1,500 |
| CARB universal cat | $380 – $680 — frequently re-throws P0420 |
FAQ
Why is P0420 common on the Honda Accord?
The Accord’s catalytic converter is precious-metal-light for cost-effective emissions compliance. By 130-150k miles the K24 4-cyl and J35 V6 cats genuinely wear out. On the 1.5L turbo (2018+), the thermostat and oil dilution issues also contribute.
How much to replace the Accord catalytic converter?
Honda OE or DENSO direct-fit: $720-$1,500. CARB universal: $380-$680 but frequently re-throws P0420 on Hondas within a year. The OE-equivalent is cheaper than two universal replacements.
Does the Honda Accord V6 exhaust manifold crack cause P0420?
Yes — Honda TSB 11-035 covers exhaust manifold cracks on 2008-2012 V6 Accords. The crack lets ambient air into the exhaust upstream of the post-cat sensor and triggers P0420 even with a healthy cat. Fix the manifold before replacing the cat.
Will a universal cat work on my Accord?
Usually not for long. Honda’s catalyst monitor has tight thresholds and most CARB-compliant universal cats trigger P0420 again within 6-12 months. Honda OE or DENSO direct-fit is the reliable choice.