Plain English
What P0420 means for your Nissan Altima
Your Camry's ECM detected that cylinder 1 isn't firing properly - the crankshaft position sensor saw the rotational speed dip slightly each time cylinder 1 was supposed to combust. On the 2.5L 2AR-FE and earlier 2AZ-FE, this almost always traces back to a tired ignition coil. Toyota's coils are durable but they have a typical service life of 80k-130k miles, and once one starts to break down it usually shows up as P0420 (or P0302/P0303/P0304) under load. The cylinder-1 coil sits at the front of the engine and runs slightly hotter than the rear coils, which is why it tends to fail first.
🎯 Top Causes on the Nissan Altima
55%
#1 MOST LIKELY
Catalyst Poisoned by Oil Consumption
The QR25DE 2.5L (2007-2012) is famous for excessive oil consumption - burning oil contaminates the cat substrate. By 100k miles, P0420 is almost expected. Check oil level monthly.
PART
$400–
🎯 Top Causes on the Nissan Altima
,500LABOR
60%
#1 CAUSE
Failing Cylinder 1 Ignition Coil
Toyota uses coil-on-plug (COP) ignition - one coil per cylinder bolted directly above the spark plug. After 80k-130k miles the secondary windings in the cylinder-1 coil break down and the spark gets weak under load. Easy diagnostic trick: swap the cylinder 1 coil with the cylinder 3 coil and clear the code. If the misfire moves to P0303, the coil is the problem. Denso and NGK make Toyota OEM-equivalent coils for around $50-$80 each. Replace all four coils as a set if mileage is high - they all age together.
OEM Coil
$80-$120
Aftermarket
$45-$70
w/Labor
$120-$280
25%
#2 CAUSE
Worn Spark Plug on Cylinder 1
The Camry's iridium plugs are spec'd for 100k miles but they wear early if oil consumption is present (especially on 2007-2009 2AZ-FE engines, which have a known oil-burning issue). A worn plug raises the firing voltage demand and overstresses the coil. Check the cylinder-1 plug for excessive gap, oil fouling, or a worn electrode. Use only Denso or NGK iridium plugs - cheap copper plugs cause repeat misfires within months on this engine.
Plugs (set 4)
$30-$60
V6 Plugs (6)
$45-$90
w/Labor
$80-$220
15%
#3 CAUSE
Fuel Injector Clog or Leak
If the cylinder-1 injector is partially clogged or leaking, the air-fuel ratio in that cylinder is wrong and you get a single-cylinder misfire that swaps when you swap injectors. Less common than coil/plug but worth checking after the easy stuff. Pull the injectors and have them flow-tested at a shop, or just replace the cylinder-1 injector with a known-good unit to confirm.
Injector
$60-$140
Flow Test
$80-$150
w/Labor
$150-$320
DIY
Medium
25%
#2 COMMON
Worn Downstream O2 Sensor
Bank 1 Sensor 2 (post-cat) wears at 90k-130k. A sluggish sensor reports false P0420 when the cat is still healthy. Always test/replace this first.
PART
$50–
🎯 Top Causes on the Nissan Altima
80LABOR
$60–
🎯 Top Causes on the Nissan Altima
20DIY
Easy
12%
#3 POSSIBLE
Wiring Chafe at O2 Sensor Connector
Heat and vibration cause the harness to chafe near the O2 sensor connector on Altimas. Corrosion or open wires cause intermittent codes that mimic cat failure.
PART
🎯 Top Causes on the Nissan Altima
0–$50LABOR
$80–
00
60%
#1 CAUSE
Failing Cylinder 1 Ignition Coil
Toyota uses coil-on-plug (COP) ignition - one coil per cylinder bolted directly above the spark plug. After 80k-130k miles the secondary windings in the cylinder-1 coil break down and the spark gets weak under load. Easy diagnostic trick: swap the cylinder 1 coil with the cylinder 3 coil and clear the code. If the misfire moves to P0303, the coil is the problem. Denso and NGK make Toyota OEM-equivalent coils for around $50-$80 each. Replace all four coils as a set if mileage is high - they all age together.
OEM Coil
$80-$120
Aftermarket
$45-$70
w/Labor
$120-$280
25%
#2 CAUSE
Worn Spark Plug on Cylinder 1
The Camry's iridium plugs are spec'd for 100k miles but they wear early if oil consumption is present (especially on 2007-2009 2AZ-FE engines, which have a known oil-burning issue). A worn plug raises the firing voltage demand and overstresses the coil. Check the cylinder-1 plug for excessive gap, oil fouling, or a worn electrode. Use only Denso or NGK iridium plugs - cheap copper plugs cause repeat misfires within months on this engine.
Plugs (set 4)
$30-$60
V6 Plugs (6)
$45-$90
w/Labor
$80-$220
15%
#3 CAUSE
Fuel Injector Clog or Leak
If the cylinder-1 injector is partially clogged or leaking, the air-fuel ratio in that cylinder is wrong and you get a single-cylinder misfire that swaps when you swap injectors. Less common than coil/plug but worth checking after the easy stuff. Pull the injectors and have them flow-tested at a shop, or just replace the cylinder-1 injector with a known-good unit to confirm.
Injector
$60-$140
Flow Test
$80-$150
w/Labor
$150-$320
DIY
Medium
8%
#4 POSSIBLE
Failed Catalytic Converter (no other cause)
If oil consumption isn't the issue and the O2 sensor is good, the cat itself has aged out. Aftermarket CARB-compliant cats run 00-$800 for QR25.
PART
00–
🎯 Top Causes on the Nissan Altima
,200LABOR
60%
#1 CAUSE
Failing Cylinder 1 Ignition Coil
Toyota uses coil-on-plug (COP) ignition - one coil per cylinder bolted directly above the spark plug. After 80k-130k miles the secondary windings in the cylinder-1 coil break down and the spark gets weak under load. Easy diagnostic trick: swap the cylinder 1 coil with the cylinder 3 coil and clear the code. If the misfire moves to P0303, the coil is the problem. Denso and NGK make Toyota OEM-equivalent coils for around $50-$80 each. Replace all four coils as a set if mileage is high - they all age together.
OEM Coil
$80-$120
Aftermarket
$45-$70
w/Labor
$120-$280
25%
#2 CAUSE
Worn Spark Plug on Cylinder 1
The Camry's iridium plugs are spec'd for 100k miles but they wear early if oil consumption is present (especially on 2007-2009 2AZ-FE engines, which have a known oil-burning issue). A worn plug raises the firing voltage demand and overstresses the coil. Check the cylinder-1 plug for excessive gap, oil fouling, or a worn electrode. Use only Denso or NGK iridium plugs - cheap copper plugs cause repeat misfires within months on this engine.
Plugs (set 4)
$30-$60
V6 Plugs (6)
$45-$90
w/Labor
$80-$220
15%
#3 CAUSE
Fuel Injector Clog or Leak
If the cylinder-1 injector is partially clogged or leaking, the air-fuel ratio in that cylinder is wrong and you get a single-cylinder misfire that swaps when you swap injectors. Less common than coil/plug but worth checking after the easy stuff. Pull the injectors and have them flow-tested at a shop, or just replace the cylinder-1 injector with a known-good unit to confirm.
Injector
$60-$140
Flow Test
$80-$150
w/Labor
$150-$320
DIY
Medium
🚗 Most Affected Camry Model Years
| Year | Engine | Primary Cause | Typical Mileage | Notes |
|---|---|---|---|---|
| 2010-2017 | 2.5L 2AR-FE | Coil pack | 80k-130k | Most common P0420 reports |
| 2007-2009 | 2.4L 2AZ-FE | Coil + oil-fouled plug | 90k-160k | Oil consumption shortens plug life |
| 2018+ | 2.5L A25A-FKS | Coil pack | 60k-90k | Newer engine, fewer reports so far |
| 2007-2017 | 3.5L 2GR-FE V6 | Rear-bank coils | 80k-130k | Rear coils harder to access |
🔧 How to Diagnose P0420 on a Nissan Altima
- Swap the cylinder 1 ignition coil with cylinder 3. Each coil is held by a single 10mm bolt and an electrical connector. Move the coil-1 to position 3 and the coil-3 to position 1, clear the code, and drive 5-10 minutes. If the misfire follows the coil to cylinder 3 (you'll see P0303), replace the original cylinder-1 coil. This 15-minute test confirms the coil 9 times out of 10 on a Camry.
- Check the spark plug. Pull the cylinder-1 plug and inspect: black sooty deposits = rich/coil; oil-wet = oil consumption (common on 2AZ-FE); white/blistered = lean. Gap should be 0.043" on most Camrys. Replace all four plugs as a set if mileage is over 80k - mixing new and old plugs causes more misfires.
- Read live data with a scanner. Look at fuel trims and the misfire counter per cylinder. If only cylinder 1 is misfiring AND fuel trims are normal, focus on ignition (coil/plug). If cylinder 1 misfires AND fuel trims are skewed, suspect injector or compression. A compression test (should be 170-220 psi on 2AR-FE) rules out a bigger mechanical issue.
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