OBD-II CODE · POWERTRAIN
P0420 — Catalyst System
Efficiency Below Threshold
Bank 1 catalytic converter efficiency is below the PCM’s expected threshold. What it means, why it triggers, and how to diagnose it correctly before spending money on parts.
// 01 — Definition
What Does P0420 Mean?
CodeP0420
Full DefinitionCatalyst System Efficiency Below Threshold (Bank 1)
SystemEmissions / Exhaust After-Treatment
SeverityModerate — drivability often normal, emissions failure common
Common Related CodesP0300–P0308, P0171, P0174, P013x/P014x, P0430
P0420 is set when the PCM compares the upstream (front) O2 sensor signal to the downstream (rear) O2 sensor signal and determines the converter on Bank 1 is not storing oxygen or smoothing exhaust changes as expected. A healthy converter produces a steadier downstream signal compared to the switching upstream signal. When both sensors switch similarly, the PCM concludes the converter is not doing its job.
// 02 — Bank Identification
What Does “Bank 1” Mean?
Bank 1 is the side of the engine that contains cylinder #1. On inline (4-cylinder) engines there is only one bank — P0420 always refers to the single converter. On V-style engines (V6/V8), Bank 1 and Bank 2 are on opposite sides. Which side is Bank 1 varies by manufacturer and engine — always confirm for your specific vehicle before purchasing parts.
V6/V8 note: If you have a V-style engine and get both P0420 and P0430 together, the issue is likely systemic (oil burning, rich condition, severe misfire) rather than two separate converter failures. Diagnose the root cause before replacing any hardware.
// 03 — Symptoms
Symptoms of P0420
Check Engine Light (most common)
Often the only symptom. The vehicle may drive completely normally while P0420 is stored.
Failed emissions / inspection readiness
P0420 will fail most OBD-II emissions tests. The catalyst monitor will show incomplete or failed.
Sulfur / “rotten egg” smell from exhaust (sometimes)
A failing converter may not fully process sulfur compounds, causing a noticeable egg-like odour.
Reduced power or sluggish acceleration (sometimes)
Only if the catalyst substrate has collapsed and is physically restricting exhaust flow.
Rattling noise from underneath (sometimes)
If the converter’s internal ceramic substrate has broken apart, it will rattle — especially on cold starts or acceleration.
// 04 — Causes
Common Causes of P0420
🔧 Catalytic Converter
Normal catalyst aging and substrate wear
Overheated or melted substrate from prolonged misfires
Contamination from oil burning or coolant in exhaust
Cheap aftermarket converter not meeting PCM threshold
📡 Oxygen Sensors
Slow or biased upstream O2 sensor
Faulty downstream sensor mimicking converter failure
Wiring, connector corrosion, or harness damage
💨 Exhaust Leaks (Very Common)
Leaks at manifold, flex pipe, gaskets, or flanges
Cracks before or near the converter
Corroded or loose O2 sensor bung
⚙️ Engine Conditions
Misfires — especially under load (destroys converters fast)
Lean running / unmetered air leaks
Rich running from injector or fuel control issues
Fix misfires first: If you have misfire codes (P0300–P0308) alongside P0420, address the misfires before any exhaust work. Active misfires send unburned fuel into the converter, rapidly overheating and destroying the substrate — a new converter will fail in the same way.
// 05 — Severity
How Serious Is P0420?
P0420 is not an immediate breakdown code — most vehicles drive normally with it stored. However it will cause emissions test failure, and if the underlying cause is a misfire, rich condition, or oil burning, continuing to drive accelerates converter damage and increases repair costs.
When to act quickly: If P0420 appears alongside misfire codes, if the vehicle runs poorly, if you smell strong fuel or sulfur from the exhaust, or if you notice a rattling from underneath — address it promptly. A collapsed converter can restrict exhaust flow enough to damage the engine.
// 06 — Diagnosis
How to Diagnose P0420
1
Scan for all codes
Retrieve stored, pending, and permanent codes. Misfires, fuel trim codes, O2 sensor codes, or EVAP codes alongside P0420 change the diagnostic path entirely.
2
Check for exhaust leaks
Inspect manifold, flex pipe, gaskets, and flanges — especially near the upstream O2 sensor. An exhaust leak introduces unmetered oxygen that confuses the PCM into thinking the converter is failing.
3
Review fuel trims
Check LTFT and STFT at idle and cruise. Persistent trims above +10% indicate a lean condition that needs fixing before any converter work. Running lean destroys new converters.
4
Evaluate O2 sensor waveforms
Upstream sensor should switch actively. Downstream should be relatively flat (steady near 0.5–0.7V) when the converter is healthy. If downstream mirrors upstream with rapid switching, the converter is genuinely failing.
5
Check for oil burning or coolant loss
Oil ash and coolant contaminate the catalyst substrate. Check the dipstick trend and coolant level. White or blue-grey exhaust smoke is a key indicator.
6
Temperature comparison test (optional)
With an infrared thermometer, compare exhaust pipe temperature before and after the converter when hot. A functioning converter runs significantly hotter at the inlet than outlet. Equal temperatures indicate the substrate has collapsed.
// 07 — Repairs
How to Fix P0420
| Repair | Most Likely When |
|---|---|
| Repair exhaust leak | Leak found at manifold, flex pipe, gasket, or near O2 sensor bung — always check this first |
| Replace downstream O2 sensor | Downstream sensor is slow, erratic, or stuck and wiring checks out — rule this out before the cat |
| Replace upstream O2 sensor | Upstream sensor is slow or biased, causing incorrect fuel control that skews the comparison |
| Fix misfires / fuel trim issues | P0420 appears with P030x misfire codes or persistent lean/rich fuel trims |
| Replace catalytic converter | All of the above are ruled out and converter efficiency is genuinely below threshold on live data |
Don’t parts-cannon this code. P0420 is one of the most misdiagnosed codes in automotive repair. Replacing a $800+ catalytic converter when the real cause was a $60 O2 sensor or a cracked exhaust flex pipe is one of the most common and expensive DIY mistakes. Follow the diagnostic steps above before ordering any parts.
// 08 — Cost Estimates
P0420 Repair Cost Estimates
| Repair | Estimated Cost Range |
|---|---|
| Exhaust leak repair (gasket/clamp/weld) | $80 – $450 |
| Downstream O2 sensor replacement | $120 – $350 parts + labor |
| Upstream O2 sensor replacement | $120 – $350 parts + labor |
| Ignition repair for misfires (plugs/coils) | $80 – $600 |
| Catalytic converter replacement (aftermarket) | $300 – $900 + labor |
| Catalytic converter replacement (OEM) | $600 – $2,500+ + labor |
Costs vary significantly by vehicle make, model, and location. See the vehicle-specific section below for make/model cost ranges.
// 09 — External References
External Resources
EPA Vehicle Emissions
Why catalytic converters matter and how OBD-II emissions monitoring works.
epa.gov →
NHTSA Recalls Lookup
Check for emissions or engine control recalls by VIN — may cover your repair for free.
nhtsa.gov →
SAE Standards Library
Industry standards and technical references behind OBD-II diagnostic protocols.
sae.org →
// 10 — FAQs
Frequently Asked Questions
No — and this is the most important thing to understand about P0420. Exhaust leaks, faulty oxygen sensors, engine misfires, lean/rich conditions, and oil burning can all trigger P0420 without the converter actually failing. Always rule out these cheaper causes before replacing a converter. The diagnostic steps above tell you what to check first.
Many vehicles drive normally with P0420 stored, but you will typically fail an emissions test. If the vehicle is also misfiring, running poorly, producing unusual exhaust smell, or you hear rattling from underneath — address it promptly. A collapsed converter that restricts exhaust flow can damage the engine over time.
P0420 refers to Bank 1 — the side of the engine containing cylinder #1. P0430 is the same fault on Bank 2. Inline 4-cylinder engines only ever set P0420 since they have one bank. Getting both codes simultaneously on a V-style engine almost always points to a systemic cause rather than two separate converter failures.
Not reliably — and on some vehicles (especially Toyota Camry) a cheap universal-fit converter will re-trigger P0420 within weeks. The PCM’s efficiency threshold is precise, and many low-cost converters don’t meet it. Use a direct-fit converter from a reputable brand (MagnaFlow, Walker, Davico) and ensure it meets CARB standards if your vehicle is California-spec.
If P0420 is the only code, the vehicle is running smoothly, and there’s no rattling or unusual smell — driving for weeks or months while diagnosing is generally fine. The risk escalates significantly if companion codes exist (especially misfires), if the vehicle runs rough, or if oil or coolant is entering the exhaust. In those cases, extended driving causes accelerating damage.
Not Sure Where to Start?
Describe your P0420 situation — your vehicle, any other codes, and what you’ve checked so far — and get a targeted diagnosis path from our free AI Diagnostic tool.
// 11 — Vehicle-Specific Notes
P0420 by Vehicle — Make & Model Notes
Vehicle-specific causes, known quirks, TSB history, and what to check first on the most commonly affected models.
Toyota Camry
Most affected: 2002–2011 (4-cyl 2AZ-FE) · V6 models also affected · #1 most reported P0420 vehicle in North America
P0420 Frequency
Very High
// Most Likely Causes
1
Catalyst wear — 2AZ-FE engine
The 4-cyl 2AZ-FE is the single most P0420-prone engine in North America. Converters degrade between 80k–130k miles, especially on vehicles that ran low on oil or experienced misfires.
2
Downstream O2 sensor (B1S2) failure
A lazy B1S2 sensor clears P0420 on a significant percentage of Camry cases before the cat is actually bad. Check waveform before replacing the converter.
3
Cheap aftermarket converter fails threshold
Camry PCMs are notably strict. Many cheap aftermarket converters trigger P0420 within weeks. Toyota OEM or CARB-compliant MagnaFlow recommended.
4
Flex pipe crack (2AZ-FE)
Known weak point between the manifold and converter. A hairline crack introduces unmetered air near the upstream sensor, causing false P0420.
// Camry-Specific Notes
📋 TSB EG013-04 — Affects 2002–2004 Camry 4-cyl (2AZ-FE). Toyota issued a PCM recalibration to loosen the efficiency threshold. Ask your dealer specifically for EG013-04 — may fix the code without any hardware replacement at ~$80–$150.
⚠️ V6 Camry: Bank 1 is the rear bank (firewall side) on 3MZ-FE and 2GR-FE engines. Confirm bank location before buying parts. Both P0420 + P0430 together usually means oil burning or lean condition — not two dead cats.
💡 Check fuel trims first: LTFT above +10% consistently = fix the lean condition before touching the exhaust. Common causes: cracked intake boot, PCV hose, vacuum leak. A lean-running Camry will destroy a new converter rapidly.
💰 Converter choice: Universal-fit converters frequently fail to clear P0420 on Camrys. Use Toyota OEM ($300–$600) or MagnaFlow California-spec equivalent (~$200–$350).
Typical Costs
Downstream O2 sensor (Denso OEM)$80–$160
Flex pipe repair / weld$120–$280
Cat converter (aftermarket CARB)$250–$500 + labor
Cat converter (Toyota OEM)$450–$900 + labor
PCM recalibration (TSB EG013-04)$80–$150 at dealer
Not sure if it’s the sensor or the cat? Describe your Camry’s symptoms and any live data readings for a targeted diagnosis.
⚡ Diagnose My CamryHonda Civic
Most affected: 1999–2005 (D/K-series), 2006–2011 (R18A/K20), 2012–2015 (R18/K24) · Inline 4-cyl only — P0420 only, never P0430
P0420 Frequency
High
// Most Likely Causes
1
Degraded OEM catalytic converter
On Civics with 120k+ miles, P0420 with no other codes and clean O2 sensor data typically means the cat is genuinely worn. Honda OEM replacement is often the right call.
2
Downstream HO2S (B1S2) wiring damage
The rear O2 sensor sits low on Civics and is vulnerable to road debris. Inspect the harness for chafing or melting before testing the sensor itself.
3
Manifold-to-head gasket leak
Common on 2001–2005 D-series Civics. A small exhaust leak introduces unmetered oxygen near the upstream sensor. Listen for ticking when cold.
4
Aftermarket intake or exhaust modifications
CAI and cat-back exhausts that alter airflow can confuse the PCM’s catalyst efficiency calculation. P0420 after a mod is often tuning-related, not hardware failure.
// Civic-Specific Notes
📋 TSB — Civic Hybrid (2003–2005 ULEV) and Insight (2002–2006) only. Non-hybrid Civics do not have a PCM recalibration TSB for P0420. If a shop says your standard Civic needs a reflash for this code, get a second opinion.
💡 Waveform tip: B1S2 closely mirroring B1S1 with rapid switching = genuine converter failure. B1S2 flat near 0.45V = cat is likely fine, check the sensor and wiring.
⚠️ California-spec Civics require CARB-compliant converters. A federal-only converter will re-trigger P0420 and fail smog. Verify your VIN’s emission standard before ordering.
💰 Honda Civic converters are affordable. Quality direct-fit (Davico, MagnaFlow, Walker) runs $120–$280. Labor typically 1–2 hours.
Typical Costs
Downstream O2 sensor (Denso/NGK)$60–$130
Exhaust manifold gasket$80–$220 incl. labor
Cat converter (aftermarket direct-fit)$150–$350 + labor
Cat converter (Honda OEM)$280–$550 + labor
Got live data from your Civic? Paste B1S1 and B1S2 voltage values for a targeted call on sensor vs converter.
⚡ Diagnose My CivicChevrolet Silverado
Most affected: 1999–2007 (5.3L/4.8L V8), 2007–2013 (5.3L AFM) · Bank 1 = passenger side · P0420 + P0430 can appear together
P0420 Frequency
High
// Most Likely Causes
1
Oil consumption — 5.3L AFM system
The 5.3L AFM/DOD engine (2007+) has documented oil consumption through the lifter system. Burning oil coats the catalyst with ash, degrading efficiency. Check oil consumption before anything else.
2
Downstream O2 sensor sluggish response
GM’s PCM comparison algorithm flags a lazy rear sensor as P0420 before it throws its own code. Swap the rear sensor before condemning the cat.
3
Cracked exhaust manifold or gasket
Silverado V8 manifolds crack at outer bolt holes — especially driver’s side 4.8L/5.3L. A crack triggers P0420 with no actual converter issue.
4
Genuine catalyst wear — 150k+ miles
With no oil consumption and clean O2 data, a high-mileage Silverado converter simply reaching end of life is the likely diagnosis.
// Silverado-Specific Notes
⚠️ 5.3L AFM oil consumption — check first: If burning oil, fix that before the converter. A new $800 converter fails within 20,000 miles if oil continues entering the exhaust. Consider an AFM delete kit.
🔍 Bank 1 = passenger side on Silverado V8s. P0420 = passenger side. P0430 = driver side. Both codes together almost always = systemic cause, not two failed cats.
💡 Manifold crack check: Start a cold engine and carefully hold your hand near each manifold bolt area. A crack produces a clearly audible tick or hiss during warmup that often disappears hot. 5-minute check before any parts purchase.
📋 Check nhtsa.gov/recalls with your VIN — several Silverado model years have emissions recalls that may cover converter repairs for free.
Typical Costs
Downstream O2 sensor (AC Delco/Bosch)$60–$140
Exhaust manifold gasket repair$200–$500 incl. labor
Cat converter (aftermarket)$250–$600 + labor
AFM delete kit (if oil burning is cause)$300–$600 + labor
P0420 + P0430 together on your Silverado? That pattern almost always means oil burning. Describe your situation for a targeted path.
⚡ Diagnose My SilveradoFord F-150
Most affected: 2004–2010 (5.4L 3V Triton), 2011–2014 (5.0L/3.5L EcoBoost), 2015–2020 (5.0L Coyote) · Bank 1 = passenger side on most F-150 engines
P0420 Frequency
High
// Most Likely Causes
1
Spark plug blowout — 5.4L 3V Triton
The 2004–2010 5.4L Triton 3-valve plug design can eject, leaving the tip in the head. The resulting misfire rapidly destroys the catalyst. Check all 8 plugs first on any 5.4L Triton P0420.
2
EcoBoost oil/coolant intrusion (3.5L)
Intercooler condensate and turbo seal leaks can contaminate the catalyst on higher-mileage 3.5L EcoBoost trucks. Check for white smoke on startup and inspect charge pipes.
3
Downstream O2 sensor failure
Ford PCM thresholds can flag a failing rear sensor as P0420 before the sensor throws its own code. Rear O2 should sit flat at 0.5–0.7V on warm idle when cat is healthy.
4
Y-pipe / flex pipe crack
Fatigue cracks in the Y-pipe and flex section are common on high-mileage F-150s, especially with off-road use. A pre-converter crack triggers false P0420.
// F-150-Specific Notes
⚠️ 5.4L 3V — check plugs first: A partially ejected plug causes a misfire violent enough to destroy a converter in one drive cycle. Use penetrating oil and the Ford-recommended removal procedure to avoid snapping plugs in the head.
🔍 Bank 1 = passenger side on 5.0L Coyote, 5.4L Triton, and 3.5L EcoBoost. Confirm with your engine’s firing order if unsure.
📋 PZEV-spec F-150s (California/Northeast states) have tighter efficiency thresholds. A converter that passes on a standard-spec truck may fail on a PZEV truck. Verify emission spec before ordering a replacement.
💡 EcoBoost P0420: Also scan for P0087 (low fuel pressure) and MAF codes. A dirty MAF or weak fuel pump can trigger catalyst efficiency codes without any converter failure.
Typical Costs
Spark plugs — 5.4L 3V all 8 (labor)$300–$700
Downstream O2 sensor (Motorcraft/Bosch)$60–$150
Y-pipe / flex pipe repair$150–$400
Cat converter (aftermarket)$350–$700 + labor
Cat converter (Ford OEM)$600–$1,200 + labor
5.4L Triton or 3.5L EcoBoost? These engines have specific P0420 root causes. Describe your engine and symptoms for a targeted path.
⚡ Diagnose My F-150Nissan Altima
Most affected: 2002–2012 (2.5L QR25DE), 2013–2018 (2.5L QR25DE/3.5L VQ35DE) · 4-cyl inline only — P0420 only. V6: Bank 1 = firewall side
P0420 Frequency
Moderate-High
// Most Likely Causes
1
QR25DE oil consumption fouling catalyst
The 2.5L QR25DE has documented oil consumption between changes. Burning oil coats the catalyst with ash over time. Check oil consumption rate before replacing any parts.
2
Upstream A/F ratio sensor (wideband)
Nissan uses a wideband A/F sensor upstream rather than narrowband O2. An incorrectly reading A/F sensor causes fueling adjustments that trigger P0420 with a healthy converter.
3
Worn catalyst — 100k–130k miles
Altima converters typically last 100k–130k under normal conditions. P0420 with clean O2 data and no companion codes at this mileage usually means genuine converter wear.
4
Exhaust flange gasket leak
The flange between the manifold converter and mid-pipe is a common leak point after 100k miles on Altimas. Introduces oxygen near the downstream sensor bung.
// Altima-Specific Notes
⚠️ QR25DE oil consumption — critical: Measure consumption over 1,000 miles before any exhaust work. More than 1qt per 2,000 miles = fix that first. A new converter fails prematurely if oil continues entering the exhaust.
🔍 A/F sensor vs O2 sensor: Nissan’s upstream B1S1 is a wideband sensor with stable linear output. Erratic or pegged readings = sensor failure, not converter failure. Check both sensors before condemning the cat.
📋 PZEV/SULEV emissions warranty: Federal law requires 8-year/80,000-mile warranty on emissions components on certified PZEV/SULEV vehicles. Check your hood sticker — if PZEV-certified and under warranty, converter repair may be free.
💡 Temperature test: With engine fully hot, use an IR thermometer to compare exhaust temp before and after the converter. Equal temps or hotter after = collapsed substrate. Converter is genuinely dead.
Typical Costs
Downstream O2 sensor (Denso/NGK)$70–$150
Upstream A/F sensor (B1S1)$100–$220
Exhaust flange gasket repair$80–$180 incl. labor
Cat converter (aftermarket)$200–$500 + labor
Cat converter (Nissan OEM)$400–$900 + labor
QR25DE Altima with P0420? Check oil consumption before buying any parts. Describe your mileage and situation for a targeted diagnosis.
⚡ Diagnose My Altima