P0420 on Kia

The 2nd generation Forte with the Nu 2.0L GDI engine (2014-2018) can develop engine knocking and excessive oil consumption. Similar to the Theta II issues but in the Nu engine family, piston ring wear leads to oil blow-by and cylinder scoring. Kia extended warranty coverage under the engine campaign. Affected engines may consume 1+ quart of oil per 1,000 miles.

Forte 2.0L Nu cars that consume oil push unburned oil ash through the exhaust, which poisons and eventually plugs the washcoat on the close-coupled (manifold) converter and the underfloor converter. Owners see P0420 catalyst-efficiency-below-threshold, sometimes with a rotten-egg smell, sluggish acceleration, and a rattle from the exhaust as the ceramic brick breaks apart. Forte owners report replacing the easy-to-reach underfloor converter only to have P0420 return within weeks because the manifold-mounted primary converter is the one that is actually degraded. In advanced cases the fractured substrate raises backpressure enough to cause power loss and stalling, and debris can be drawn back toward the head.

Theta II, Nu, Gamma, Kappa, and Lambda engines in Kia Optima vehicles consume excessive amounts of oil between service intervals. This issue is often linked to the bearing debris problems which grind into piston rings and gouge cylinder walls. Many owners report using 1 quart or more of oil every 1,000 miles with no visible leaks.

The 2.4L GDI Sorento (2011-2016) experiences premature catalytic converter failure, often linked to the Theta II oil consumption issue. Excess oil burning fouls the catalytic converter substrate, causing it to overheat and break down internally. Symptoms include a sulfur smell, reduced power, and P0420/P0430 codes. On vehicles with excessive oil consumption, the root cause must be addressed first or the replacement converter will fail again.

P0430 only applies to the V6 Sorento, since Bank 2 (the cylinder bank containing cylinders 2-4-6) exists solely on the 3.3L and 3.5L Lambda II V6 (2011-2020). The ECM sees the upstream and downstream oxygen-sensor signals tracking too closely on that bank, meaning the rear (post-cat) catalytic converter is no longer scrubbing enough pollutants. On the Sorento this is usually an aged/degraded catalytic converter or a lazy rear (downstream) O2 sensor on Bank 2, and occasionally an exhaust leak upstream of the sensor that skews the readings. There are rarely drivability symptoms — the illuminated check-engine light and a failed emissions test are typically the only signs.

NHTSA recall 19V120000 (Kia SC176) covers 2012-2016 Soul vehicles with the 1.6L GDI engine. Excessive exhaust-gas temperature can damage the catalytic converter, leading to abnormal combustion, piston damage, connecting-rod failure, stall or fire. Kia’s later SC176Y bulletin addresses certain previously repaired vehicles with P0420, with or without a P030X misfire code.

NHTSA recall 21V259000 (Kia SC209) covers certain 2020-2021 Soul vehicles with 2.0L Nu MPI engines whose piston oil rings may not have been properly heat-treated. Kia communication 11017169/SST082 indexes a later PNSS vibration-inspection process to certain 2020-2023 Soul vehicles and describes ring-related cylinder-wall damage, increased oil consumption, abnormal noise, oil-pressure warning and P1327. That later bulletin does not make every 2022-2023 vehicle part of recall 21V259.

The catalytic converter on the Spectra can fail prematurely, often before 100,000 miles. The catalyst substrate deteriorates or becomes clogged, causing reduced engine performance and increased emissions. Oil burning from worn valve seals can contaminate the catalyst, accelerating failure. The Spectra was covered under the federal emissions warranty (8 years/80,000 miles) for catalytic converter failure, but many vehicles are now beyond this coverage.