I have a 2006 M45 with 120k miles. It had a problem where it wouldn't start after driving somewhere then trying to restart the car a few minutes to an hour later. It would crank but not fire. Then a few minutes later it would start and run fine. I thought the fuel pump was dying so replaced it, but the problem persisted. I eventually scanned for codes and, though there was no SES light, there was a pending code that pointed to the camshaft position sensor, so I replaced that and the initial problem went away, but in its place I got P0420 and P0430. The SES light was never on before.
Thinking the new fuel pump's pressure might be too high, I replaced it with used OEM. That didn't fix anything. I replaced both downstream O2 sensors and then got P2a00 and P2a03. The battery was weak so I replaced it, which I read can sometimes resolve weird codes that don't appear to have a cause, but there was no change (other than the battery doesn't die after being parked a few days).
Last weekend, I replaced the upstream sensors but the P2a00 came back. The P2a03 showed as pending but cleared after driving again, while the P2a00 was actually logged.
I used Torque Light to log the O2 sensor voltage output over a ~15 minute drive on city streets and the freeway (screenshot attached; yellow arrow shows lowest reading and red shows highest). It appears to me that Bank 1 is running lean while Bank 2 is running rich. It also looks like the catalytic converters are working properly based on the downstream O2 sensor readings.
My latest thought is that when I replaced the fuel pump, some debris got into the line and was pumped into one or more fuel injectors, causing those cylinders to run lean, and the ECU is adding fuel to compensate. I've ordered refurbished OEM injectors, which will be here by the end of the week.
Gas mileage might be suffering a little. This car was never fuel efficient. Idle and acceleration seem fine and it doesn't overheat.
Am I interpreting the O2 sensor readings correctly?
What else could cause one bank to run lean while the other runs rich? I assume anything north of the intake, such as the plastic snorkel to the air filter box, would affect both banks equally.
Are there other likely causes of P2a00 and P2a03?
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