What does code P2187 mean?

P2187 means the mixture runs lean specifically at idle on bank 1 — the classic small-vacuum-leak signature. It’s moderately serious — you can usually keep driving gently, but diagnose it soon.

What does P2187 mean in plain words?

The mixture runs lean specifically at idle on bank 1 — the classic small-vacuum-leak signature. A leak is a fixed amount of air, so it dominates at idle: go straight to the vacuum-leak hunt (intake boot, hoses, PCV, purge).

How is this different from P0171/P0172?

Scope: P0171-family means the correction maxed out overall; P2187-family means specifically at idle. The idle-only version usually has a smaller, more findable cause — treat it as a head start.

Why does my idle also surge with this code?

Same root: the computer chasing a mixture it can't balance at low airflow. Fix the lean/rich cause and the surge typically leaves with it.

P2187 — System Too Lean at Idle (Bank 1)

Moderate

Quick answer

P2187 means the mixture runs lean specifically at idle on bank 1 — the classic small-vacuum-leak signature. First move: a leak is a fixed amount of air, so it dominates at idle: go straight to the vacuum-leak hunt (intake boot, hoses, PCV, purge).

What it means

P2187 reports that the mixture runs lean specifically at idle on bank 1 — the classic small-vacuum-leak signature.

These codes are the fuel-trim verdicts scoped to idle only — and that scoping is the diagnostic gift. A mixture problem that exists only at idle is dominated by fixed-quantity causes: a vacuum leak (fixed extra air, huge at idle, diluted at speed) or a leaking injector/purge valve (fixed extra fuel, same math in reverse).

The steps below run cheapest-first — because in this family, the cheap causes really are the common ones.

P2187 symptoms: what you'll notice

Rough or fluttery idle, especially sitting at stoplights with the engine warm.
Occasional stalling or near-stalls when you roll to a stop.
A hissing or whistling sound from the engine bay at idle — the vacuum leak announcing itself.
Idle speed that surges up and down as the computer fights the lean mixture.
Driving above idle often feels completely normal — the problem hides at low RPM.

Common causes

Ordered from most to least likely.

1.

See the diagnosis steps

This family shares its suspect list; the steps walk it in cost order.
2.

Wiring or connector damage

The universal suspect for circuit-flavored codes.
3.

The component named by the code

Condemned by measurement, never by guess.

How to fix it: diagnosis, step by step

Cheapest and most likely checks first.

1 Confirm the idle-only pattern

Watch long-term fuel trim at idle versus at 2,500 RPM. Big correction at idle that fades with RPM is the fixed-quantity signature this family exists to report.
2 Lean codes: hunt the leak

Intake boot, vacuum hoses, PCV, intake gaskets, purge line — the full hunt is our vacuum leak guide, spray test and all.
3 Rich codes: find the fuel

Pinch the purge line at idle (trims snap back = purge valve), check fuel pressure, and consider a leaking injector — plug reading finds the wet cylinder.

Parts & tools you may need

Disclosure: some links are affiliate links (including the Amazon Associates program). If you buy through them, we may earn a commission at no extra cost to you.

Related codes

Frequently asked questions

What does code P2187 mean?: P2187 means the mixture runs lean specifically at idle on bank 1 — the classic small-vacuum-leak signature. It’s moderately serious — you can usually keep driving gently, but diagnose it soon.
What does P2187 mean in plain words?: The mixture runs lean specifically at idle on bank 1 — the classic small-vacuum-leak signature. A leak is a fixed amount of air, so it dominates at idle: go straight to the vacuum-leak hunt (intake boot, hoses, PCV, purge).
How is this different from P0171/P0172?: Scope: P0171-family means the correction maxed out overall; P2187-family means specifically at idle. The idle-only version usually has a smaller, more findable cause — treat it as a head start.
Why does my idle also surge with this code?: Same root: the computer chasing a mixture it can't balance at low airflow. Fix the lean/rich cause and the surge typically leaves with it.