What it means
Fuel trim on bank 1 hits the lean limit only at idle, while trim returns to normal at higher load. This pattern strongly indicates a vacuum leak — leaks become a larger fraction of total airflow at idle and so dominate the trim, but disappear into background noise above idle.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The engine control module uses bank 1 oxygen-sensor feedback to adjust fuel delivery. P2187 sets when bank 1 fuel trim reaches the lean limit at idle but returns toward normal as engine airflow increases; that idle-specific pattern points toward unmetered air entering through a vacuum leak or a PCV valve stuck open.
Common causes
Most likely first.
- Vacuum leak (intake gasket, brake booster, hoses)
- PCV valve stuck open
Symptoms
- Check engine light on
- Rough or unstable idle
- Possible hesitation
How to diagnose it
- Record freeze-frame data and compare bank 1 fuel trims at idle and at a higher engine speed; trim that improves off idle supports an idle vacuum-leak pattern.
- With the engine off, inspect bank 1 intake-gasket areas, vacuum hoses, and brake-booster hose for cracks, loose connections, or damage.
- Check the PCV valve and its hoses for damage or a valve that remains open, which can admit excess air at idle.
- If the visual checks do not locate a leak, use an appropriate intake leak test to pinpoint unmetered air around the bank 1 intake and vacuum connections.
- After repairing any leak, recheck idle fuel trims and confirm they are within the manufacturer's specification.
Common fixes
- Reconnect a loose vacuum hose or replace a split hose feeding the bank 1 intake.
- Replace a PCV valve that is stuck open and repair damaged PCV hoses.
- Repair a leaking bank 1 intake gasket or brake-booster hose after confirming the leak.
- Clear the fault and verify that bank 1 fuel trims return to the manufacturer's specification at idle.
Can I keep driving?
Short, gentle trips are generally possible if the engine remains controllable, but arrange a repair soon because the rough idle may worsen or cause hesitation. Avoid relying on the vehicle if it stalls or hesitates severely, especially in traffic.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.