o2. Sensor Rear bank upstream code P2240

**megatrack** · 02-25-2018, 09:34 PM

I am a BMW newbie .I just bought last week a 325i 2004 wagon with 71k miles. I have code P2240 which means according to what I read in the forum an O2 sensor rear bank upstream. So there are 2 in the front and 2 in the back? Should I replace all of them at once or just the rear top one.
Thank you for helping.

**MarcoZandrini** · 02-26-2018, 07:28 AM

A bit of info first. BMW splits it engines with 6 or more cylinders into banks. Your car has 6 cylinder M54B25 engine. The engine family is M54 and the engine is a B25 2.5l variant. The M54 engine came in various displacements up to 3l, the M54B30. The engine has 2 banks, banks 1 & 2. Cylinders 1-3 are bank 1, while cylinders 4-6 are bank 2. Each bank has 2 oxygen sensors. One is before the catalytic converters, the other is after the converter. So, the engine has 4 O2 sensors. The pre-converter sensors are referred to as “up stream,” while the post converter sensors are referred to as “down stream.” The code, P2240, refers to the pre-converter sensor for bank 2. The cause could be several things. The first thing to do is to visually inspect the wiring for the sensors to determine if it’s intact. As a side note, has anyone recently replaced the valve cover gasket? I ask because the O2 sensor wires can get caught between the valve cover gasket and the valve cover. If the wiring looks ok, the next step is to remove the black plastic cover that sits over the intake manifold. The O2 sensor wires connect to the engine harness there. THE FIRST STEP IS TO LABEL ONE PAIR OF CONNECTORS SO YOU DONT MISCONNECT THE SENSORS. That is, reconnecting the sensors to the engine harness and connect sensor 1 to the sensor 2 engine connector. You’ll be chasing problem forever!! So, you need o make sure that the sensor’s “oxygen sensor” circuit is not broken in the sensor. How? Use an ohm meter. Be advised that there are actually 2 circuits in an O2 sensors!! One is the O2 sensor. The other is the heater circuit. A heater circuit? Yup. This circuit is energized during cold starts to get the O2 sensircupnyo temoerature quickly.