Plain English
What P0420 means for your Chrysler 300
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 Chrysler 300
50%
#1 MOST LIKELY
Broken Exhaust Manifold Bolts (5.7L Hemi)
The Hemi studs corrode and snap at 80k+ on Chrysler 300s - same defect as Ram and Charger. Leak fakes P0420. Fix the bolts first.
PART
$50–00
LABOR
$400–
🎯 Top Causes on the Chrysler 300
,200DIY
Hard
25%
#2 COMMON
Aged Downstream O2 Sensor
Bank 1 Sensor 2 fails at 100k. Diagnose with scan tool live data before any cat work.
PART
$60–
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
LABOR
$80–
🎯 Top Causes on the Chrysler 300
50DIY
Easy
15%
#3 POSSIBLE
3.6L Pentastar Cat Substrate Failure
On Pentastar V6 300s, the OEM cat substrate cracks at 120k+. Replacement is the only fix.
PART
$400–
🎯 Top Causes on the Chrysler 300
,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
10%
#4 POSSIBLE
MDS Lifter Misfire Cooking Cat
Hemi MDS lifter failure causes misfires that destroy the cat. Fix MDS first.
PART
,000–$6,000
LABOR
(included)
DIY
Hard
🚗 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 Chrysler 300
- 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.
Want a step-by-step walkthrough specific to your Chrysler 300? Run a $5.99 AI diagnosis report - we narrow the cause to your year, engine, and symptoms.