Rough engine running Seat Leon 1M, engine code BCB

Translated on 07-07-2026, 8:54.

Good day.

I have a Seat Leon 1M with engine code BCB that has a very unstable idle and occasional misfires.
So far, I have cleaned and reset the throttle body, replaced the EGR valve, replaced both oxygen sensors, replaced the charcoal canister valve, and replaced the spark plugs and ignition coils. Do you have any advice for me? Occasionally, the error message "Mixture in area 2 below lean limit" appears in the error memory, but there is no check engine light (MIL).
I'm slowly at a loss. The vacuum pressure sensor was replaced in December.

"Lately, I've been experiencing a slight jerking, especially when I suddenly press the accelerator, but it only happens occasionally. And the error memory is clear." Can someone help me?

Translated on 07-07-2026, 8:56.

Quote:

If the engine is running too lean and the oxygen sensor is measuring correctly, then...
because the fuel pressure is okay (high enough), but the injector(s) are not.
constipated?

Best regards, Rainer

Translated on 07-07-2026, 8:56.

My Seat is now experiencing both jerking during driving and a P0171 error message. The fuel pressure regulator was replaced a year ago.

Translated on 07-07-2026, 8:58.

Quote:

The fuel pressure regulator was replaced a year ago.

And that's why the fuel pressure should be okay now, or what?

The entire time, your engine control unit (ECU) is telling you that the engine is running too lean. So, check why he's not getting enough fuel!
Does this engine have a mass airflow sensor? If so, it's possible that unmetered air is being drawn in behind it, which would then not be measured. For combustion to occur, there is more air present than what is measured, which consequently results in the mixture running too lean.

If you're not planning to check the intake system, fuel pressure, and fuel injector, we can stop here, right?

Translated on 07-07-2026, 8:59.

The intake system has been checked. I removed the fuel injectors and they were tested; everything is okay. The fuel pressure at idle is normal. My suspicion is that the problem, which always occurs after a period of acceleration, is related to a drop in fuel pressure. This would also explain why you can hear a gurgling sound in the tank if you lie underneath the vehicle when the car is turned off. So, it seems it depends on the pressure, as they suspected. Thank you very much.