I just got a 2005 Jeep Grand Cherokee (WK) with the Hemi engine and 88,000 miles. I already love it. After driving it for about two days, I started it up to leave my neighborhood, and I heard a rattling or light metal banging noise coming from the Jeep. I only hear it when accelerating or slowing down, but not when it’s stopped.
I turned off the Jeep and used the key trick to check for error codes. It showed:
P0420 Catalyst System Efficiency Below Threshold (Bank 1)
I know catalytic converters can be really expensive, especially for a Jeep with a Hemi engine. I’ve also noticed that the Jeep’s gas mileage is really bad, averaging about 10 mpg in the city. I haven’t driven it long enough to calculate the exact mileage.
Could this issue be caused by a bad catalytic converter, EGR valve, or O2 sensors? I can borrow a friend’s scanner tool to check live data, but I’m not sure what to look for.
What have other WK owners with Hemi engines experienced with this issue?
Unless the code is active, you might not have any problems. Sometimes a P0420 code can pop up randomly and go away on its own. If you clear the code, it might not come back.
Your car checks the values of the upstream and downstream oxygen (O2) sensors. The P0420 code appears when the downstream sensor readings are off. However, this test happens briefly during each drive, so something small could have triggered the code. That’s why I usually clear the code first to see if it comes back. If it does, then I do more testing.
Catalytic converters (cats) don’t fail often. It’s more common for the O2 sensors to become weak or faulty.
To check, warm up your car and then run the engine at about 2000 RPM. Using a scanner, the upstream O2 sensors (Bank1Sensor1 and Bank2Sensor1) should show a wave pattern between 0 and 0.8 volts. These numbers help with fuel control. Bad gas mileage can sometimes mean weak upstream O2 sensors.
For the catalytic converter, look at the downstream O2 sensors (Bank1Sensor2 and Bank2Sensor2). These should show slight movement, around 0.5 volts or 0.0 volts. If it looks mostly flat, your catalytic converter is fine. If the downstream sensors are erratic and follow the upstream sensors closely, the catalytic converter might be bad (the inner part is likely damaged).
(A blocked catalytic converter can cause a lack of power.)
Is there a way to clear the code without a scan tool? Disconnecting the battery feels like overkill for just one code. I’ve been using an Autel scanner that my friend is kind enough to let me borrow, but I still need to figure out how to erase a code with it.
I’ve had the catalytic converters go bad on a couple of cars, and they made a light rattling sound. That’s the material inside breaking apart. Sometimes you can hear the rattle by tapping on the converter, but it depends on how damaged it is. Some of these cars also had poor performance because the converter was so clogged.
If the converter is bad, don’t just replace it right away. It’ll just get clogged again unless you figure out what’s causing the problem (trust me, I learned the hard way).
In my case, one car was running too rich because of a vacuum leak. It ruined two Magnaflow converters in a year and a half, and I had only driven about 1,500 miles. If only one of your converters is bad, focus on that side of the engine. You might get lucky and just need to change the spark plugs or replace a bad coil.
I’ve cleared the error codes with the scanner, and the P0420 and P0520 codes haven’t come back yet after two trips—one to get lunch and the other on my way home from work.
This paycheck, I’ll buy a new EGR valve, engine pressure sensor, and spark plugs for the Hemi. Hopefully, this will improve my gas mileage and get it into the mid-teens for city driving.
If the P0420 code comes back, I’ll start by replacing the O2 sensors. The Denso sensors (which seem to be the original manufacturer) are only $38 at O’Reilly’s, which is cheaper than I expected. As a last resort, I’ll replace the catalytic converter, which is about $175 from RockAuto—not as expensive as I thought!
Is the engine loud, like there might be an exhaust leak? I had bolts break on one of the downpipes, which caused an exhaust leak and might have been the reason I kept getting those P0420 codes.
Usually, a bad catalytic converter (cat) or aftermarket cats aren’t great. But if the P0420 code comes back and you find out the cat is bad but not falling apart, and you’re tired of replacing it, you could just use spark plug non-foulers and be done with it, depending on your location and emission testing requirements.
I was able to run my CJ7 without a catalytic converter because it’s 30 years old and exempt from emissions laws in Texas. But this newer vehicle has a lot more electronic sensors and data going to the PCM. I don’t want to run the WK without a cat, especially since emissions rules are getting stricter in many states.
EDIT: I had to look up what non-fouler plugs are. How do they fix the P0420 code, and doesn’t it just avoid fixing the real problem if there is one?