P2187: System Too Lean at Idle (Bank 1)
P2187 means the engine computer is adding more fuel than allowed at idle and low load on bank 1 because the air-fuel mixture is too lean (too much air, too little fuel). The most likely cause is an air leak after the air mass sensor, so the car is usually drivable for a short time but should be checked soon.
You can drive, but get it checked soon
The data does not list limp mode for this code, so the car normally keeps running. A lean mixture at idle can still cause rough running and extra emissions, and the cause (such as an air leak) tends not to fix itself, so have it checked soon rather than driving on it for weeks.
What it means
Bank 1 is the side of the engine that contains cylinder 1. The engine computer constantly corrects the fuel amount, and the slow, learned correction is called the long-term fuel trim. Many systems, especially Bosch ones, split this correction into an additive part, which dominates at low airflow such as idle, and a multiplicative part, which dominates under load. An air leak adds a fixed amount of air that the air mass sensor never measures. That extra air matters most at idle, so the additive correction grows until it passes its limit and the code is set. The limits differ by manufacturer and engine. For example, some GM 3.6 V6 engines set the code above roughly 6 to 7 percent of added fuel. Your check engine light is on, and this code counts as emissions-related.
Likely causes
- Air leak after the air mass sensor, often at the crankcase ventilationVery likely
Unmeasured air entering the intake, for instance through the crankcase ventilation parts, leans out the mixture most strongly at idle.
- Damaged front oxygen or A/F sensor wiringLess likely
Damaged or shorted wiring can make the sensor report a wrong mixture, and in one documented case brake fluid spilled on the wiring caused a short.
- Fuel pressure too lowLess likely
If the fuel system delivers too little fuel, the computer has to add more to compensate and reaches its limit.
What you may notice
- Check engine light on
- Rough or hunting (rising and falling) idle
- Hissing sound from the engine bay
What to check, cheapest first
- 1Listen for a hissing sound at idle and look for loose, cracked or disconnected air hoses, especially around the crankcase ventilation hoses and valve. · you can do it
- 2Check that the oil filler cap and dipstick are seated properly and the intake hose connections after the air mass sensor are tight. · you can do it
- 3Read the stored codes and fuel trim values with a scan tool, and check for related codes such as P0171, P2189 or P2279. · you can do it
- 4Do a smoke test of the intake and crankcase ventilation system to find leaks that are not visible. · mechanic
- 5Inspect the front oxygen or A/F sensor wiring for damage, chafing or contamination such as spilled fluid. · mechanic
- 6Measure fuel pressure and compare it with the specification for the engine. · mechanic
Related codes
Frequently asked questions
Can I drive with P2187?+
Usually yes for a short time, since limp mode is not listed for this code. Get it checked soon, because a lean idle can cause rough running and higher emissions.
Can a loose gas cap cause P2187?+
This code is about extra air or too little fuel at idle, and the gas cap is not among the listed causes. An air leak after the air mass sensor is far more likely.
Will P2187 pass an emissions test?+
Not with the check engine light on. The code is emissions-relevant, so the cause needs to be repaired and the light off before testing.
Can I fix it myself?+
Often yes. The repair is rated medium difficulty and DIY is possible, with an estimated 0.5 to 2.5 hours, mostly for finding and fixing an air leak.
Written with the help of AI from the public OBDex definition (CC0). Each make may use the code differently. Updated: 2026-11-10