P0420 on Dodge: How to Diagnose & Fix

On HEMI V8s, the bolts that hold the cast-iron exhaust manifolds to the cylinder heads can break over time. Some manifolds also crack. Once a bolt breaks, exhaust escapes at the manifold-to-head joint and you hear a sharp tick. The tick is loudest on a cold start and often fades as the engine warms up, because the hot metal expands and closes the gap. Owners of 5.7L Ram, Durango and Jeep models with the same engine family call it a very well-known problem. One owner found several broken bolts on each side, and reports cover both the passenger and driver sides. People often mistake the noise for the "HEMI tick" from the lifters. One Challenger source says exhaust leaks are fairly cheap to fix and do not cause catastrophic engine damage. A repair-cost guide warns that a leak left for a long time can turn into a more expensive job.

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 has also issued PCM software bulletins for P0420/P0430 — for example TSB 18-090-24 (superseding 18-086-19) for 2019 Challengers with the 5.7L HEMI, which names PCM software as the cause — 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 can burn excessive oil; FCA's warranty-extension bulletin 09-003-23 lists the cause as the interactive deceleration fuel shut-off calibration. 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). The settlement extended the Powertrain Limited Warranty to 7 years/100,000 miles from the in-service date for oil-consumption repairs (XB1 warranty extension); CSN W80 provides a free oil consumption test and long block replacement if the engine fails.

The plenum pan gasket on the underside of the intake manifold 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.