[NavB58]

Hey there,

I recently purchased a 2018 BMW 440i with 140,000km (85,000km) and had a check engine light pop up. Car is stock, has no modifications or tune.

Code: P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

The car has Lubrico warranty and is going for a diagonstics next week, just curious if anyone with a B58 has ran into this same code? I am hearing that it is most likely a failing cat, or o2 sensors. I can't really see the cat giving up this early.

BMW Canada has a emissions warranty for 130,000km / 8 years, I am slightly over the kilometres but hopping they can help as well.

Please let me know if you ran into the same issues,
Thank you!

[F36carman]

I just got the same code at 60k miles. Mine also is a non-modified car. What was your result?

[Billfitz]

https://www.walkerexhaust.com/suppor...0420-code.html

[NavB58]

Sorry to hear about that... I would personally recommend the following:

Techincally you are under warranty for the cataylic system, for your Federal Emissions Warranty. It covers upto 8 years or 80,000 miles. I would contact BMW and take the car to them. Someone else on this forum had a new free cat installed.

Here are some other options;


1. Clear the code - and see if the code comes back. I seen in the forums that people do get this code from time to time, and it usually doesn't come back.

2. If the code comes back, take it to the shop. I ran some basic diagonstics, and seen that the code had a misfire as well. Sparkplugs were changed and that fixed my issue..


or I thought. Unfortunately month later, with the car being tuned to stage 1 I recieved the same code. My mechanic shop ran tests, road test etc and couldn't find anything wrong with the car. They recommend I put a catless downpipe and flash the car stage 2. That's whats happening to the car now as we speak!

[AlmondJayRoM3]

Originally Posted by NaveenF30

Sorry to hear about that... I would personally recommend the following:

Techincally you are under warranty for the cataylic system, for your Federal Emissions Warranty. It covers upto 8 years or 80,000 miles. I would contact BMW and take the car to them. Someone else on this forum had a new free cat installed.

Here are some other options;


1. Clear the code - and see if the code comes back. I seen in the forums that people do get this code from time to time, and it usually doesn't come back.

2. If the code comes back, take it to the shop. I ran some basic diagonstics, and seen that the code had a misfire as well. Sparkplugs were changed and that fixed my issue..


or I thought. Unfortunately month later, with the car being tuned to stage 1 I recieved the same code. My mechanic shop ran tests, road test etc and couldn't find anything wrong with the car. They recommend I put a catless downpipe and flash the car stage 2. That's whats happening to the car now as we speak!

What kind of shop can’t figure out a standard P0420 code? Worse what kind of shop recommends going catless to get rid of a P0420 code?

Process is really simple. Make sure no misfire codes or spark codes(which they did) Test for bad O2 sensors. If those are good verify catalytic converter performance by data logging O2 sensor response and comparing upstream and downstream sensor output to changing conditions. It will quickly reveal how well your cats are working.

[NavB58]

Originally Posted by AlmondJayRoM3

What kind of shop can’t figure out a standard P0420 code? Worse what kind of shop recommends going catless to get rid of a P0420 code?

Process is really simple. Make sure no misfire codes or spark codes(which they did) Test for bad O2 sensors. If those are good verify catalytic converter performance by data logging O2 sensor response and comparing upstream and downstream sensor output to changing conditions. It will quickly reveal how well your cats are working.

The car was checked by a BMW Indy Shop & my local BMW dealership the second time around.

No parameter issues on road tests, and both he upstream and downstream showed no signs of a weak/failing cataylic convertor. The car showed no other issues while logging as well. They recommend I clear the code, and keep driving. - BMW asked if I wanted to replace the cat & O2 sensors and quoted me close to 3k CAD.

I already intended to go catless, TU Fuel Pump and stage 2, just not within two months of owning the car. but seemed to make sense to get rid of the code.

[zahidramazan]

The P0420 code indicates that the catalytic converter efficiency is below the normal threshold. There are a few possible causes for this:

- Catalytic converter failure - The converter may be damaged or worn out, reducing its ability to properly process exhaust gases. Replacing the faulty converter is the typical repair.

- Exhaust leak - A leak before the catalytic converter allows unburned gases to bypass the converter, reducing its effectiveness. Inspect exhaust pipes and gaskets for any leaks.

- Engine running rich - Too much fuel in the mixture can overload the converter and reduce its efficiency. Check for dirty fuel injectors, faulty oxygen sensors, or issues with the mass airflow sensor.

- Oil burning - Excess oil being burned in the engine can coat the catalyst inside the converter, causing loss of efficiency. Perform a compression test to check for worn piston rings or cylinders allowing oil to be burned.

- Bad oxygen sensor - A faulty post-cat oxygen sensor can provide incorrect readings back to the PCM causing the efficiency code. Replace the oxygen sensor.

- Underlying engine issue - Problems like a misfire, bad spark plug, or faulty ignition coil can cause unburned fuel to reach the converter, reducing efficiency. Diagnose and repair underlying engine issues.

Start by scanning for any other codes in addition to P0420, as this can help indicate where to begin diagnosing. Checking converter operation along with engine performance is key to resolving the root cause of the efficiency code.