P2187: System Too Lean at Idle Bank 1
What to do
Expect a rough, uneven or hunting idle that clears once you are moving. This code says the problem is at idle specifically, which points at air leaking into the engine after the airflow sensor rather than at anything to do with fuel supply. Vacuum leak testing is inexpensive and is the right first step; a fuel pump is not. Mention it if the brake pedal has felt harder than usual, as a leaking brake servo produces both.
What this code means
Bank 1 runs lean specifically at idle, and the module has recorded that condition rather than a general lean state.
Lean at idle and nowhere else is close to a diagnosis on its own. The arithmetic is under P2177: a leak admits a roughly fixed quantity of unmetered air, and idle is where the engine's own airflow is smallest, so that fixed quantity is the largest proportion of the total it will ever be. A leak too small to notice at 3,000 rpm can be several per cent of the air entering at idle.
So this code points hard at unmetered air — air reaching the cylinders without passing the airflow sensor — and the places it gets in are a short and well-known list: intake manifold gaskets, the crankcase ventilation system and its hoses, brake servo hose and diaphragm, throttle body gasket, injector seals on a direct-injection engine, and any of the small vacuum lines that perish with age and heat.
Two of those deserve singling out. A brake servo leak is often missed because the symptom is looked for in the engine bay while the leak is behind the bulkhead, and it can produce a firmer brake pedal that nobody connects to a mixture code. And a PCV system is designed to admit air — so a stuck-open valve or a split hose is not an obvious hole, it is a metered path that has stopped being metered.
A note on what this code is not. Fuel starvation almost never presents this way. An engine idling needs very little fuel, so a weak pump, a blocked filter or a failing regulator will meet idle demand comfortably and fail only under load — which is the opposite pattern, and is P2191. Replacing a fuel pump for a lean-at-idle code is the classic wasted repair in this family.
Likely causes
Start here
- Vacuum leak in intake manifold or hoses · moderate
- Leaking intake manifold gasket · moderate
- Faulty PCV valve or split hose · easy
- Disconnected or leaking vacuum hose · easy
Less common
- Leaking brake servo or servo hose · moderate
- Leaking throttle body gasket · easy
- Faulty oxygen sensor · moderate
Symptoms
Components involved
Information, not instruction
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