OBD codes / P0420 / Jeep Grand Cherokee
P0420 on a Jeep Grand Cherokee
Catalyst System Efficiency Below Threshold (Bank 1)
3.6L Pentastar V6 and 5.7L HEMI V8, WK2 (2011–2021) and later
What P0420 usually means on a Grand Cherokee
On the Grand Cherokee, P0420 means the downstream oxygen sensor is seeing too much leftover oxygen behind the bank 1 catalytic converter, so the PCM judges the cat is no longer scrubbing exhaust efficiently. On high-mileage Pentastar V6 and HEMI V8 trucks it usually reflects a genuinely aged converter. But it can also be triggered upstream by a small exhaust leak, an oil-burning cylinder, or a lazy front O2 sensor fouling the substrate early. Because the converters sit close to the manifolds, ruling out leaks and sensor drift first can save an expensive part.
Most likely causes on this model
- Aged bank 1 catalytic converter
Original converters on 100k-plus Pentastar and HEMI Grand Cherokees lose efficiency, especially if the engine ever ran rich or burned oil.
- Exhaust leak ahead of the converter
Leaks at the manifold or Y-pipe pull outside air past the sensor and mimic a dead cat; common as gaskets and bolts age.
- Lazy or contaminated upstream O2 sensor
A slow-switching front sensor skews fuel trims and can set P0420 even when the converter is still healthy.
- Oil consumption fouling the substrate
A Pentastar burning oil or a HEMI with a valvetrain issue can coat the cat and shorten its life.
What it costs on a Grand Cherokee
A quality direct-fit converter plus an oxygen sensor typically runs several hundred to well over a thousand dollars installed, depending on engine and whether one or both banks are affected. Labor is moderate because the converters are reachable from underneath. California-emissions trucks narrow your legal parts choices and push the price up, since cheaper universal cats often will not keep the code off.
Fixing it
Before condemning the converter, have a shop smoke-test the exhaust and watch live O2 data; a manifold gasket or a drifting front sensor is a far better first spend than a cat that fails again for the same underlying reason.
For the full step-by-step diagnosis order (any vehicle), see the main P0420 guide.
Grand Cherokee owners also ask
Will replacing the rear oxygen sensor fix P0420 on my Grand Cherokee?
Occasionally, if that sensor is genuinely lazy, but P0420 specifically grades catalyst efficiency, not sensor health. Most of the time the rear sensor is accurately reporting a worn cat, so verify with live data before buying parts.
Is it safe to keep driving with P0420?
Mechanically yes, it will not strand you, but the truck will fail emissions and a failing cat can slowly worsen fuel economy. Address it before an inspection deadline.
Year, engine and mileage change the answer. Get a diagnosis for your Grand Cherokee — it weighs these causes against your exact configuration and real complaint data.
Diagnose my Grand Cherokee free →Other codes we cover on the Jeep Grand Cherokee
Related symptoms
Jeep Grand Cherokee problems & recalls by year
Based on widely reported patterns for this model; individual causes vary by year, engine and history. Generic P0420 behavior is covered on the main P0420 page. Confirm with a scan tool before buying parts.