P0420 on Dodge: How to Diagnose & Fix

Exhaust manifold bolts break due to thermal cycling causing the bolts to become brittle and snap, especially on the passenger side. Dissimilar metals (iron manifold, aluminum head) accelerate the process. Creates a ticking noise at cold startup. Common across all HEMI-powered LX/LC platform vehicles.

A P0420 ("Catalyst System Efficiency Below Threshold, Bank 1") or P0430 ("Bank 2") check-engine light is one of the most common emissions codes on the Dodge Challenger, especially on higher-mileage V8 cars. The PCM sets the code when it compares the upstream (pre-cat) and downstream (post-cat) oxygen-sensor signals: if the downstream sensor begins to mimic the rapidly-switching upstream sensor instead of staying relatively flat, the computer concludes the converter is no longer storing/releasing oxygen efficiently. On the Challenger this code is frequently NOT a worn-out converter at all. The two most common upstream root causes are (1) an exhaust leak from broken exhaust-manifold bolts — the well-documented "HEMI tick" on the 5.7L and 6.4L V8s — which lets atmospheric oxygen into the exhaust stream and confuses the rear O2 sensor, and (2) a long-running misfire (worn plugs, failing coils, leaking injectors on the 3.6L Pentastar) that dumps raw fuel into the converter and physically destroys it over time. A failed downstream O2 sensor, contaminated converter (from an oil-cooler/oil-filter-housing leak on the Pentastar), or simply an aged/degraded cat round out the causes. Chrysler also issued software bulletins (TSB 18-021-12, superseded by 18-033-13) because early PCM calibration logic was overly sensitive and could trigger false P0420/P0430 codes — particularly in warm weather or on long highway drives — so a PCM reflash should be ruled out before any hardware is replaced. Because the converter sits in the federal/CARB emissions-warranty window for many of these cars, the diagnosis order matters a lot to the owner's wallet.

The 2.4L Tigershark MultiAir II engine has defective piston rings allowing oil into combustion chamber. Owners report 1 quart per 1,000-2,000 miles. FCA considered up to 1 quart per 2,000 miles 'acceptable.' Class-action settlement reached (Wood, et al. v. FCA US, LLC). CSN W80 warranty extension: 7 years/100,000 miles.

The plenum gasket between the intake manifold and engine block fails, allowing engine oil to be sucked into combustion chambers. Causes excessive oil consumption, pinging, and visible oily sheen inside intake manifold through throttle body. Common on all 5.2L and 5.9L Magnum V8 engines.

The plenum gasket between intake manifold and engine block fails, allowing oil into combustion chambers. Same issue as 1st-gen Durango. Causes oil consumption, pinging, and rough idle.

The Dodge Stealth (particularly RT/TT and ES models) features an Active Exhaust System with electronically controlled valves that open at higher RPM to reduce backpressure. These valves seize from rust and carbon buildup, causing restricted exhaust flow, reduced power, and a check engine light. The vacuum-actuated system uses solenoids and diaphragms that also fail with age. Many owners simply remove the system entirely and replace with a straight-through exhaust, which provides better performance and eliminates the failure point. The AES is shared with the Mitsubishi 3000GT as both cars were built on the same platform.