P2187: System Too Lean at Idle (Bank 1)
Key takeaways
- P2187 means the engine computer has had to add too much fuel on bank 1 at idle and has hit its limit.
- At idle, a small air leak is a big share of the air the engine takes in, so vacuum leaks at the PCV system, hoses or intake gasket are the most common cause.
- It is usually safe to drive for a few days, but stalling, a very rough idle or a flashing light means get it checked now.
- A smoke test finds most idle-lean faults for the price of a diagnostic hour, $100–$180, before anyone buys a part.
What P2187 means
Your engine is getting more air than its fuel accounts for, but only at idle, and the computer has run out of room to add fuel. Scanners call it System Too Lean at Idle (Bank 1). The upstream oxygen sensor reports lean exhaust, the computer adds fuel through its fuel trim, and when the trim stays at its limit while the engine idles, P2187 sets. Bank 1 is the side of the engine with cylinder 1; on a four-cylinder that’s the whole engine.
Idle matters because the engine takes in very little air there. A leak that would barely register at highway speed can be a big share of the total at idle.
Is it safe to drive?
Yes, for a few days in most cases. Many cars with P2187 feel normal or idle a little rough. Book a diagnosis soon anyway: vacuum leaks tend to grow, and a lean engine runs hotter. The answer changes if the engine stalls at lights, the idle hunts up and down badly, or the check engine light flashes. Those mean it’s lean enough to misfire, which can damage the catalytic converter, so drive only as far as a shop.
Causes, ranked
| Cause | How you confirm it | Typical cost (US independent shop, 2026) |
|---|---|---|
| Cracked vacuum or PCV hose, loose oil cap or dipstick | Hissing at idle; fuel trims high at idle, near normal at 2,500 rpm; smoke test | $50–$400 (most hoses at the low end) |
| PCV valve stuck open or split PCV diaphragm | Whistle near the valve cover; strong suction at the oil filler at idle | $50–$150 (more where the valve is built into the valve cover) |
| Intake manifold gasket or intake boot leak | Smoke escapes at the manifold joint or boot | $50–$400 (gasket at the top) |
| Dirty mass airflow sensor | Airflow reading low at idle; trims improve after cleaning | $20–$60 to clean; $150–$400 to replace |
| Exhaust leak ahead of the upstream oxygen sensor | Ticking at cold start near the manifold; soot at a joint | $150–$600 |
| Leaking or dirty fuel injector, weak fuel pressure | Lean on one bank only on a V engine; injector balance or pressure test | $250–$600 per injector |
Weak fuel pumps usually show lean under load rather than only at idle, so they come after everything above.
What to check first
- Check the easy stuff yourself. Make sure the oil filler cap is tight and the dipstick seated, then listen around the top of the engine at idle for a hiss.
- Read every code. P2187 with P0171 says bank 1 is lean everywhere; with P0174 or a bank 2 idle code, look for something both banks share, like the PCV system or the intake.
- Compare fuel trims at idle and 2,500 rpm. High at idle and near normal at higher rpm is the vacuum-leak pattern.
- Smoke test the intake and PCV system. It finds leaks you can’t hear.
- Clean the airflow sensor with dedicated MAF cleaner before replacing it.
- Clear it and drive. If the code stays off for a week of mixed driving with plenty of idling, the fix worked.
The bulletins point the same way. Jaguar traced some F-Pace cases to a split diaphragm in the pressure control (PCV) valve in the right-hand cam cover, and Land Rover has a Discovery bulletin for fuel trims near their limit at idle. GM told technicians to check the turbocharger-to-converter joint for exhaust leaks on the 2007 Pontiac Solstice and Saturn Sky, and Mazda traced some 2007–2009 CX-7 cases to a damaged air-fuel sensor harness cover.
What it costs
The cheap outcome is a tightened oil cap, a cleaned airflow sensor at $20–$60, or a $50 hose. The common outcome is a PCV valve or a vacuum leak repair, $50–$400. The expensive outcome is an exhaust leak at the manifold or turbo, up to about $600, or a valve cover with a built-in PCV valve. A smoke test and a fuel trim check, $100–$180, stop a shop from replacing an oxygen sensor that was only reporting the truth.
Related codes
P2187 often shows with P0171, and sometimes with P2177, the same lean condition off idle. P2188 is the opposite, rich at idle. If the airflow sensor is at fault you may also see P0101. 3 complaints filed with NHTSA mention P2187; those are owner reports, not confirmed causes.
What owners report to NHTSA
3 complaints filed with NHTSA mention P2187, 2 of them since 2015, across 3 different models. The component owners most often filed it under: fuel system, gasoline.
Manufacturer service bulletins that name P2187
7 technical service bulletins and manufacturer communications filed with NHTSA mention P2187, from 6 makes. A bulletin means the manufacturer has documented a known cause or a fix procedure for that model. Examples:
- LAND ROVER DISCOVERY (2017, 2018, 2019, 2014), bulletin 10158996: The Check Engine Lamp is on and the vehicle is running rough. DTCs vary, may include P2187-00, P2189-00, P013E-00,P014E-00. Fuel trims near limit when engine is at idle speed.
- JAGUAR F-PACE (2017, 2018, 2019, 2014), bulletin 10159477: Air leak from the right hand camshaft cover assembly due to a split diaphragm within the pressure control valve. Any of the following DTCs (indicating an air intake system leak) could be stored in the Powertrain Control …
- NEW FLYER XE40 (2017, 2018, 2019), bulletin 10159477: Air leak from the right hand camshaft cover assembly due to a split diaphragm within the pressure control valve. Any of the following DTCs (indicating an air intake system leak) could be stored in the Powertrain Control …
- MAZDA CX-7 (2007, 2008, 2009), bulletin 10097317: SOME VEHICLES MAY EXHIBIT A LACK OF POWER WITH MIL ILLUMINATION AND DTCS P2187 / P2177 STORED IN MEMORY. P2187: FUEL SYSTEM TOO LEAN AT IDLEP2177: FUEL SYSTEM TOO LEAN AT OFF IDLETHIS CONCERN MAY BE CAUSED BY A/F SENSOR …
- PONTIAC SOLSTICE (2007), bulletin 10156959: This document is written to suppliment the P2187 diagnosis already in the service manual. This directs the technician to carefully inspect the turbo charger and catalytic converter exhaust mating surfaces for signs ofexh…
- SATURN SKY (2007), bulletin 10156959: This document is written to suppliment the P2187 diagnosis already in the service manual. This directs the technician to carefully inspect the turbo charger and catalytic converter exhaust mating surfaces for signs ofexh…
Sources: NHTSA Office of Defects Investigation complaint database and manufacturer communications file, snapshots September 2026. A complaint or bulletin that mentions a code is a report about that vehicle, not a statement that the code has one cause.
Frequently asked questions
Can I drive with P2187?
Usually for a few days, yes. Most cars idle a little rough or feel normal. Book a diagnosis soon, because a vacuum leak tends to grow. If the engine stalls at stops, idles very rough or the light flashes, stop driving it except to a shop.
What is the most common cause of P2187?
A vacuum leak. At idle the throttle is almost closed, so even a small crack in a PCV hose, a split intake boot or a tired intake gasket lets in a large share of unmeasured air. The computer adds fuel to make up for it until it runs out of room.
What's the difference between P2187 and P0171?
Both mean bank 1 is lean. P2187 sets only at idle; P0171 is judged across all driving. Lean at idle but normal at cruise is the classic vacuum-leak pattern, while lean everywhere points more toward the airflow sensor or fuel supply.
Can a bad PCV valve cause P2187?
Yes. A PCV valve that sticks open, or a torn diaphragm in engines that build the PCV valve into the valve cover, lets extra air into the intake at idle. Listen for a whistle or hiss near the valve cover and check whether the oil cap is hard to pull off at idle.