P0420 on Chrysler: How to Diagnose & Fix
Catalyst System Efficiency Below Threshold (Bank 1)
1 Chrysler model · 1 documented issue · $400-$1,500 repair
P0420 on Chrysler by model: symptoms, diagnosis and fix
The 5.7L HEMI is notorious for exhaust manifold bolts breaking, especially on the passenger side. Thermal cycling causes bolts to become brittle and snap. This creates a ticking noise at cold startup that partially diminishes as engine warms but never fully goes away. Broken bolts can be extremely difficult to extract if snapped flush with the cylinder head.
Common Symptoms
- Ticking or exhaust leak noise at cold startup
- Noise diminishes as engine warms but doesn't fully disappear
- Visible exhaust soot around manifold flanges
- Failed emissions inspection
- Exhaust smell in engine bay
How to Fix
Extract broken bolts using ProMAXX drill template tool kit (PMXA200PROP) to safely extract without damaging cylinder head. Replace with upgraded Grade 8 double-ended studs and nuts instead of bolts. Apply anti-seize compound on new hardware. Consider Hooker Blackheart stainless replacement manifolds for a permanent upgrade on high-mileage vehicles.
Sources for this issue (2)
P0420 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0420 mean on a Chrysler?
P0420 stands for "Catalyst System Efficiency Below Threshold (Bank 1)." P0420 means the engine computer determined the catalytic converter on Bank 1 isn't cleaning the exhaust as efficiently as it should. The computer compares the upstream and downstream oxygen sensors; a healthy converter stores and releases oxygen, so the downstream sensor should read fairly steady, while a worn one lets its readings mirror the upstream sensor. When that efficiency falls below a set threshold, this code is set. While it most often points to a worn-out catalytic converter, it can also be triggered by upstream problems like a faulty oxygen sensor or an exhaust leak, so the converter isn't always the actual fault. On Chrysler it is documented across 1 model: 300.
What are the most common causes of P0420?
The most common causes of P0420 are: Worn-out or failing catalytic converter (Bank 1), Faulty or aged downstream (or upstream) oxygen sensor, Exhaust leak before or near the oxygen sensors, Engine running rich or lean (fuel trim issues), Misfires or oil/coolant contaminating the converter, Damaged or aftermarket low-quality catalytic converter, Faulty oxygen sensor wiring or connectors, Software/calibration needing update (on some vehicles). The specific cause varies by vehicle.
How much does it cost to fix P0420 on a Chrysler?
Published repair estimates on Chrysler range from $400 to $1,500 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.