P0420 Code: Honda CR-V (2017-2023) - Causes, Symptoms & Fixes

No NHTSA complaints found for this make/model/year/issue combination.
Official Recall status: No recalls found in NHTSA database.

CODE MEANING AND SEVERITY

• Code: P0420
• Full description: Catalyst System Efficiency Below Threshold (Bank 1)
• What it means for your 2017-2023 Honda CR-V:
  • The PCM (engine computer) has detected that the catalytic converter located on Bank 1 is not effectively reducing pollutants as expected. This is determined by comparing the signal from the upstream oxygen sensor (before the cat) and the downstream oxygen sensor (after the cat).
  • In most CR-Vs, Bank 1 corresponds to the single bank of cylinders in the 4-cylinder engine; the downstream sensor after the cat is the one used to judge catalyst efficiency.
• Severity and implications:
  • Emissions-related fault that can cause the Malfunction Indicator Lamp (check engine light) to come on.
  • The car may still be drivable, but fuel economy can drop, and the vehicle may fail emissions testing.
  • If the underlying issue isn’t addressed, the catalytic converter could be damaged over time or continue to underperform.
• Important context:
  • P0420 is often caused by issues other than a dead catalytic converter (eg, exhaust leaks, faulty O2 sensors, misfires, or engine running conditions that contaminate the cat). A proper diagnosis is necessary to confirm the root cause.

COMMON CAUSES ON HONDA CR-V

• Faulty catalytic converter (most common cause): age or contamination can degrade catalytic efficiency.
• Exhaust leaks or pressure leaks around the exhaust manifold, pipes, or the cat inlet/outlet before or after the cat. Leaks can skew downstream O2 readings.
• Faulty downstream oxygen sensor (Bank 1 Sensor 2) or wiring: bad readings can falsely indicate low cat efficiency.
• Faulty upstream oxygen sensor (Bank 1 Sensor 1) or related wiring: improper upstream signal can create misleading fuel trims that affect downstream readings.
• Engine running condition causing catalyst contamination or over-fueling:
  • Chronic misfires (P0300-series), oil/fuel/coolant entering the combustion chamber, or very rich/lean conditions.
  • Worn piston rings, valve seals, or injector issues that allow oil or fuel impurities to reach the catalytic converter.
• Mass Air Flow (MAF) sensor or intake air leaks: incorrect air/fuel calculations can burden the cat.
• Short trips or failure to allow the cat to heat to operating temperature: catalysts don't always work efficiently when not hot enough.
• Aftermarket exhaust or modifications: non-OEM parts, unsealed joints, or mismatched sensors can cause readings to be wrong.
• Contaminated catalyst from coolant/oil or other substances (less common but plausible in engines with leaks or failures).

SYMPTOMS

• Illumination of the Check Engine Light (MIL) and a stored P0420 code.
• Often no obvious driveability problem, but possible symptoms include:
  • Reduced fuel economy
  • Slight loss of power or throttle response in some driving conditions
  • Occasional misfire codes or rough idle if other issues are present
  • In some cases, no noticeable symptoms other than the MIL
• Other related indications may appear if a second issue is present (for example, P030x misfires, P0171/P0174 fuel trims, P0421 in some engines during specific conditions).

DIAGNOSTIC STEPS

Important: Start with the simplest, least invasive checks first, and verify that the underlying issue isn’t misfire or vacuum/fuel issues that can trigger P0420 indirectly.

• Step A: Confirm code and status

  • Use an OBD-II scanner to confirm P0420 is current (not just a history code).
  • Check freeze-frame data and any related codes (P030x, P0171/ P0174, P0421, etc).

• Step B: Visual inspection

  • Inspect the exhaust system for obvious leaks, corrosion, damaged piping, loose clamps, or holes in the exhaust near the manifold, catalytic converter, or downstream sensors.
  • Inspect wiring and connectors for Bank 1 Sensor 2 (downstream O2 sensor) and Bank 1 Sensor 1 (upstream O2 sensor).
  • Look for oil or coolant leaks that could contaminate the exhaust or catalytic converter.

• Step C: Check for misfires and engine health

  • Scan for misfire codes (P0300, P0301, P0302, P0303, P0304). If present, address those first (spark plugs, ignition coils, injectors, compression, etc).
  • Check for abnormal fuel trims (Long-Term Fuel Trim) on Bank 1; large positive trims can indicate a lean condition, large negative trims a rich condition.

• Step D: Test oxygen sensors

  • With engine at operating temperature, observe live data for Bank 1 Sensor 1 (upstream) and Bank 1 Sensor 2 (downstream).
  • Upstream sensor (Sensor 1): should swing between approximately 0.2–0.9 V as the engine runs through rich/lean cycles.
  • Downstream sensor (Sensor 2): should oscillate less than upstream and, once the cat heats, be relatively steady around mid-range (roughly 0.45 V) with less dramatic swings. If downstream sensor tracks upstream sensor signals closely, catalytic efficiency may be poor.
  • Check sensor heater circuits (resistance/voltage) to ensure sensors heat quickly to operating temperature.
  • If either sensor is sluggish or sluggish to respond, consider replacing the sensor(s) before condemning the catalytic converter.

• Step E: Check for exhaust leaks and perform a leak test

  • A smoke test or pressurized air test can reveal leaks before the cat, around the cat, or at joints that could bias sensor readings.
  • Fix any leaks discovered.

• Step F: Rule out contamination or oil/coolant intrusion

  • If oil, coolant, or fuel contamination is suspected (smell, blue smoke, or other symptoms), address the source (piston rings, valve seals, injector seals) before replacing the cat.

• Step G: Heat cycle and drive cycle

  • Most catalytic efficiency monitors require a proper drive cycle to heat the catalytic converter to operating temperature and to complete readiness tests.
  • Drive the vehicle at highway speeds for several miles after the cat is warmed up, incl. some steady-state speeds to allow the catalyst to reach operating temperature.

• Step H: Decide on the repair path

  • If tests indicate a faulty downstream O2 sensor, replace it and recheck.
  • If a misfire or vacuum/fuel issue is present, fix that first and recheck P0420.
  • If the cat is suspected and readings remain inconclusive after sensor fixes, consider a cat efficiency test using the vehicle’s data and, if needed, professional confirmation.

• Step I: After repair, clear codes and perform drive cycle

  • Clear the codes and run the vehicle through a complete drive cycle to ensure the catalyst efficiency monitor completes and P0420 does not reappear.
  • If P0420 returns after reputable repair and sufficient heat cycles, revisit the diagnosis.

RELATED CODES

• P0420: Catalyst System Efficiency Below Threshold (Bank 1) — primary code discussed here.
• P0421: Catalyst System Efficiency Below Threshold (Bank 1) – Warm Up (less common on CR-V but possible in some ECU logic; relates to catalytic performance during warm-up phase).
• P0430: Catalyst System Efficiency Below Threshold (Bank 2) — typically relevant to V6 or engines with two banks; not common on the CR-V 4-cylinder, but technically a related category if the vehicle has variants with dual banks.
• O2 sensor related codes often seen with P0420:
  • P0138, P0139, P0130 (upstream sensor) or P0138/P0141 (heater or sensor circuit)
  • P0140, P0141 (downstream sensor)
• Fuel trim and misfire related cues that can accompany P0420:
  • P0171 (System Too Lean)
  • P0174 (System Too Lean, Bank 2 if applicable)
  • P0300–P0304 (random/misfire on cylinders)
• Note: On a typical 2017-2023 Honda CR-V with a single Bank 1, P0420 is the core code to address; P0421/P0430 are related in the broader OBD-II category but may not apply to every engine variant in this model range.

REPAIR OPTIONS AND COSTS (2025 PRICES)

Prices vary by region, shop, and whether you use OEM or aftermarket parts. The ranges below reflect typical U.S. prices in 2025 and assume standard labor rates. Always obtain a written estimate before work.

• Oxygen sensor replacement (Bank 1 Sensor 2 downstream, often the quickest fix)

  • Parts: $60–$250 (OEM may be higher; aftermarket sensors cheaper)
  • Labor: $50–$150
  • Typical total: $110–$400
  • Note: If the upstream sensor is also failing, replacement cost can rise to $200–$600 for two sensors.

• Catalytic converter replacement (most significant repair if the cat is failing)

  • Parts: Aftermarket catalytic converter for CR-V can run roughly $350–$900; OEM converter can be $800–$1,800 or more depending on the exact cat assembly and model year.
  • Labor: $200–$700
  • Typical total: $550–$2,500 (smaller shops using aftermarket parts on the low end; dealers with OEM cats on the high end)
  • Important note: In circumstances where the cat is still covered by a driveability warranty or emissions warranty, costs could be reduced by warranty coverage (check with Honda and your warranty status).

• Exhaust leak repair (pre-cat or around cat)

  • Parts: $20–$150 for gaskets, clamps, hangers; pipe sections may be more
  • Labor: $100–$300
  • Typical total: $120–$450

• MAF sensor, fuel injector cleaning or replacement (if suspected contributing issues)

  • MAF sensor replacement: $120–$350 (parts and labor)
  • Cleaning services: $0–$100 (DIY cleaning kits)

• Other related repairs that could be needed if root cause is misfire or contamination

  • Spark plugs, ignition coils: $100–$400 per set (parts and labor)
  • Oil/coolant leak repair: varies widely based on source

• Quick diagnostic fee

  • Some shops charge a diagnostic fee if no repair is performed; ranges from $60–$150, sometimes waived with repair.

DIY VS PROFESSIONAL

• DIY-friendly tasks (if you have the right tools and a safe workspace)
  • Read/clear codes with a basic OBD-II scan tool, observe live data for upstream and downstream O2 sensors.
  • Visual inspection of the exhaust system for obvious leaks, damaged wiring, or loose connections.
  • Replace a faulty downstream O2 sensor (Bank 1 Sensor 2) if you are comfortable with replacing exhaust sensors and ensuring proper torque.
  • Check and fix intake/vacuum leaks (PCV hose, vacuum lines, MAF sensor cleaning with proper spray) if indicated.
  • Test drive to confirm the vehicle returns to normal after sensor replacement.
• When to call a professional
  • If the catalytic converter is suspected or you need to confirm the cat’s actual efficiency (requires proper tools and experience).
  • If diagnostics indicate misfires or combustion-related issues that require advanced inspection (compression test, injector flow test, ignition system diagnostic, leak testing).
  • If exhaust system removal/replacement is required, or if the vehicle is under warranty or requires OEM parts to avoid warranty voids.
• Tools and safety
  • Proper PPE, jack/stands or lifts, torque specs for O2 sensors (typically around 22–30 ft-lbs, but verify for your model).
  • Smoke machine or pressure test equipment for exhaust leaks if available.

PREVENTION

• Regular maintenance helps prevent P0420 from becoming a recurring issue:
  • Use high-quality fuel and keep up with scheduled maintenance (oil changes, spark plugs, ignition coils, air filter, PCV).
  • Address misfires promptly; a persistent misfire can cause unburned fuel to reach the catalytic converter and damage it.
  • Maintain the exhaust system; repair leaks immediately to avoid upsetting O2 sensor readings.
  • Replace oxygen sensors at the manufacturer’s recommended intervals or when diagnostic data indicate they are failing (common intervals are 60k–100k miles, but check your owner’s manual and service bulletin).
  • Avoid short trips that do not allow the catalytic converter to heat to its efficient operating temperature; during winter, combine short trips into longer drives when possible.
  • If you modify the exhaust system (downpipe, cat-back, etc.), ensure the modification is compatible with the vehicle’s emissions system and that O2 sensors are properly calibrated.
  • If the vehicle uses coolant or oil that is leaking into the exhaust, fix the root cause (engine or cooling system issue) to prevent catalyst contamination.

Data limitations and notes

• Data status: No NHTSA complaints found for this make/model/year/issue combination.
• Data status: No recalls found in NHTSA database.
• These data points indicate a lack of documented complaints/recalls in the provided dataset, but do not guarantee absence of issues in the real world. Always use a professional diagnosis to confirm conditions before spending on repairs.
• P0420 can be caused by multiple issues beyond a failed catalytic converter; a methodical diagnostic approach is essential to avoid unnecessary replacement of the cat.

If you’d like, tell me your mileage, engine variant (if you know it), any codes you’ve pulled besides P0420, and your region. I can tailor a step-by-step diagnostic plan and give you a more precise cost estimate for your specific situation.