Catalyst System Check Engine

[chief]

Ok Bimmer mechanics here is one i need help with understanding what i have to R&R.

I am getting a P0420 code on my OBD II Diagonostic Tester following the illumination of the Check Engine light on the dash.

Code PO 420 refers to: "Catalyst System Efficiency Below Threshold (Bank 1)"
What the heck do i have to R&R to fix this problem?

As always, i value your inputs.

 

[Kirby]

Here - Google is your friend!

The code is actually P0420 (P Zero) not PO420 (P Oh)

This is part of the standard OBD codes, that is, it's the same for ANY car, it's not a BMW specific code.

P0420 OBD-II Trouble Code

Technical Description

Catalyst System Efficiency Below Threshold (Bank 1)

What does that mean?

Basically this means that the oxygen sensor downstream (after) the catalytic converter on bank 1 is detecting that the converter is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

Symptoms

You will likely not notice any drivability problems, although there may be symptoms.

Causes

A code P0420 may mean that one or more of the following has happened:

• Leaded fuel was used where unleaded was called for
• An oxygen sensor is not reading (functioning) properly
• The engine coolant temperature sensor is not working properly
• Damaged exhaust manifold / catalytic converter / exhaust pipe
• Retarded spark timing

Possible Solutions

The simplest thing to do is to reset the code and see if it comes back.

Next step is to measure the voltage at the oxygen sensor on Bank 1 (the rear sensor, or the sensor after the converter). The voltage should be low and steady (~200mV, depends on the vehicle). If the voltage varies (e.g. 100mV to 600mV) then it's time to replace the catalytic converter. In fact, it would be a good idea to test each oxygen O2 sensor while you're at it.

One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.

 

[chief]

Thanks,
Never have used leaded fuel. The O2 sensor could be fouled due to an earlier problem. The engine coolan temperature sensor is a new one on me. Might involve "retarded spark timing" however. Will be checking the O2 sensor first.

Thanks for the detailed reply. Gives me an idea where to look.

By the by, the code doesn't come up every time i drive the car. It's just serving as an occational headache.