P2187⛽ Fuel and air meteringStop and fix promptly

P2187: System Too Lean at Idle Bank 1 (Meaning, Causes, Fixes)

Reviewed 2026-10-24 · Based on 2 cited source(s)

What P2187 means

The PCM uses several sensors to work out how much fuel the engine needs and keeps adjusting the mixture to stay balanced. RepairPal says P2187 sets when the PCM can't achieve the desired balance between fuel and air. OBD-Codes.com (Don Bowman, ASE) says the code means the computer has recognized a lean mixture at idle during the last two starts. On a 4-cylinder engine 'bank 1' doesn't matter; on a V6 or V8 it's the side with cylinder number one, and P2189 is the same fault on bank 2. OBD-Codes.com calls it an ambiguous code with a long list of possible causes, and notes that if a test drive shows no symptoms it may not be a real lean condition, with the oxygen sensor or computer responsible instead. The sources differ in emphasis: OBD-Codes.com focuses on unmetered air entering the intake and low fuel pressure, while RepairPal also lists damaged or contaminated spark plugs, fuel that doesn't meet the maker's requirements, and PCM software that needs updating, and names spark plug and mass airflow sensor replacement as common repairs. P2187 is a generic OBD-II code; P0171 is a related lean code for bank 1 that isn't limited to idle.

Severity: can I keep driving?

Stop and fix promptly

Get this diagnosed soon. RepairPal rates P2187 'Urgent'. OBD-Codes.com doesn't give a severity rating but lists a miss and very irregular idle, hard hot starts and a fuel smell among the possible symptoms. If you smell raw fuel, see leaking fuel, or the engine idles so roughly it may stall, don't keep driving it; have it checked first.

Common symptoms

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

  1. Vacuum or intake air leaks after the mass airflow sensor: cracked or loose hoses, intake manifold, brake booster hose, PCV, turbo bypass, or EVAP hoses (both sources list vacuum leaks)
  2. Loose or leaky oil filler cap or a bad gas cap seal (OBD-Codes.com)
  3. Faulty front oxygen sensor or its heater (OBD-Codes.com)
  4. Faulty mass airflow (MAF) or MAP sensor (OBD-Codes.com; RepairPal lists engine sensor failure)
  5. Low fuel pressure from a weak fuel pump, clogged fuel filter, or faulty fuel pressure regulator (OBD-Codes.com; RepairPal lists fuel pump failure)
  6. Leaking or failing fuel injector (both sources)
  7. EVAP canister purge valve (OBD-Codes.com)
  8. Exhaust leaks (both sources)
  9. Faulty variable camshaft timing (OBD-Codes.com)
  10. Damaged or contaminated spark plugs (RepairPal)
  11. Fuel that doesn't meet the manufacturer's requirements (RepairPal)
  12. PCM software that needs an update, or a faulty PCM (both sources)

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

  1. Scan for other codes and fix those first; OBD-Codes.com says other lean-causing faults in the loop will often set their own codes.
  2. Make sure the gas cap and the oil filler cap are tight and their seals aren't cracked (OBD-Codes.com).
  3. Check the intake duct from the mass airflow sensor to the intake manifold for cracks or loose clamps, and look over every vacuum hose, including the brake booster hose, MAP sensor hose, PCV hose and turbo hoses (OBD-Codes.com).
  4. Listen for a whistling or hissing sound at idle that points to a vacuum leak (OBD-Codes.com).
  5. Look for external fuel leaks at the injectors (OBD-Codes.com). If you see or smell leaking fuel, stop and get it repaired before driving.

Typical fixes

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

  • Smoke testing or carefully spraying around the intake manifold, EGR base and gaskets with the engine running to find a leak; OBD-Codes.com says idle speed rises when the spray hits a leak (flammable, so leave it to a pro if unsure)
  • Fuel pressure test at idle and 2500 rpm against the factory spec (OBD-Codes.com)
  • Checking the fuel pressure regulator vacuum hose for fuel, which means the regulator has failed (OBD-Codes.com)
  • Reading live fuel trim and oxygen sensor data, and checking for technical service bulletins or PCM updates with a factory-level scan tool (OBD-Codes.com mentions a Tech 2 for the remaining checks)

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Is it safe to drive with P2187?

RepairPal rates it 'Urgent'. Many cars still drive, but a lean idle can cause a miss, rough idle and hard hot starts. Get it diagnosed soon, and don't drive it if you smell or see leaking fuel or the engine is close to stalling.

What's the most common cause of P2187?

Neither source ranks a single cause first, but both list vacuum leaks, and OBD-Codes.com says to start with unmetered air entering the intake (cracked hoses, a loose oil cap, intake leaks) or low fuel pressure.

What's the difference between P2187 and P2189?

They mean the same thing on different sides of the engine. P2187 is bank 1, the side with cylinder 1, and P2189 is bank 2 on V6 and V8 engines.

How much does it cost to fix P2187?

Neither source gives a repair cost. The fix can be anything from tightening a cap or replacing a vacuum hose to a sensor or fuel pump, so get it diagnosed and ask for a written estimate.

Sources

  1. OBD-Codes.com: P2187 System Too Lean at Idle (Bank 1) (Don Bowman, ASE)
  2. RepairPal: P2187 System Too Lean at Idle (rated Urgent)

Last reviewed 2026-10-24. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.