P2187 — Engine running too lean at idle on bank 1

The engine is detecting too much air and not enough fuel at idle on the first bank of cylinders. This can happen due to vacuum leaks, faulty sensors, or fuel delivery problems. It causes rough idling and can affect performance but usually won't cause immediate damage if addressed soon.

Common causes

The cause is almost always one of a few things. Here they are — P2187 rarely comes from anything outside this list:

• Vacuum leaks near intake manifold or hoses
• Dirty or faulty mass airflow sensor
• Weak or clogged fuel injectors
• Faulty oxygen sensor on bank 1
• Low fuel pressure or failing fuel pump

Where to start

In the order a sensible mechanic would work — the cheap checks first, the expensive replacements only after:

1. Inspect and repair vacuum leaks
2. Clean or replace mass airflow sensor
3. Test and clean fuel injectors
4. Check and replace oxygen sensor if needed
5. Test fuel pressure and replace pump if failing

That is what holds for P2187 in general. From here it depends on your engine — tell me which car it is and what it does, and I will give you the steps for it: real procedures, torque figures and diagrams instead of general advice.