P2189
System Too Lean at Idle Bank 2P2189 is a generic OBD-II powertrain diagnostic trouble code: System Too Lean at Idle Bank 2. It is logged by the engine control unit when the scr/adblue monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P2189 means
P2189 is an SAE generic powertrain code indicating that the fuel system on Bank 2 is running lean specifically at idle. The ECM monitors short-term and long-term fuel trim (STFT/LTFT) on each bank independently. P2189 is set when the Bank 2 LTFT at idle reaches the maximum enrichment limit — meaning the ECM has added as much fuel as it can but still cannot bring lambda to stoichiometry at idle speed. It is the Bank 2 sibling of P2187, and the idle-specific counterpart to P0174 (Bank 2 lean at all loads).
The idle-specific nature of this code is a key diagnostic clue. A lean condition that appears only at idle and disappears under load almost always points to a vacuum leak or an air metering issue. At idle, throttle is nearly closed; any unmetered air entering downstream of the MAF sensor represents a proportionally large fraction of total airflow and causes a significant lean spike. Under load with the throttle open wide, the same leak represents a smaller fraction of total airflow and may not be enough to push fuel trims to the threshold.
Bank 2 is the bank of cylinders that does not contain cylinder 1 — on most longitudinally mounted V-engines this is the passenger side (US market). Identifying the correct bank for the vehicle being diagnosed is essential before beginning hardware inspection.
Common causes
Most-frequently reported root causes when P2189 is logged.
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1
Vacuum leak on Bank 2 intake runners or near Bank 2 injectors (cracked hose, failed PCV fitting, split intake boot)
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2
Dirty or failed Mass Air Flow (MAF) sensor under-reporting airflow, causing lean trims system-wide (both banks)
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3
Weak or failed fuel injectors on Bank 2 not delivering sufficient fuel at low pulse widths
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4
Low fuel pressure at idle (weak fuel pump, clogged filter, leaking fuel pressure regulator)
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5
Bank 2 upstream O2 sensor failed lean-biased, causing ECM to see false lean signal
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6
Clogged EGR passages affecting Bank 2 idle mixture
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7
Air leak at Bank 2 exhaust upstream of sensor (falsely leans the sensor reading)
Symptoms drivers notice
How to diagnose P2189
A typical diagnostic flow when this code is present.
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1
Read all codes and freeze-frame data; confirm the code was set at idle (low RPM, low load in freeze frame) and check whether Bank 1 trims are also elevated
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2
Check Bank 2 LTFT and STFT at idle with a scan tool — values above +10% confirm the lean condition; values above +25% indicate a significant fault
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3
Perform a propane enrichment or smoke test around Bank 2 intake manifold gaskets, PCV hoses, throttle body, and vacuum lines to find unmetered air entry
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4
Check MAF sensor readings against a known-good specification at idle (typically 2–7 g/s on a 4-cylinder, proportionally higher on V8) — a low MAF reading with high trims on both banks points to the MAF
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5
Measure fuel pressure at idle and during cranking to confirm it holds within spec; check injector pattern and pulse width on Bank 2 cylinders
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6
Inspect the Bank 2 upstream O2 sensor for lean-stuck voltage (constantly below 0.3 V); swap with Bank 1 sensor if in doubt to see if the lean reading follows the sensor
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7
Clear adaptive fuel trims after repair and perform an idle re-learn if required by the manufacturer before confirming the fix
Related powertrain codes
- B0001 — PCM Discrete Input Speed Signal Error
- B0004 — PCM Discrete Input Speed Signal Not Present
- C0359 — Four Wheel Drive Low Range (4LO) Discrete Output Circuit
- C0362 — 4LO Discrete Output Circuit High
- P2000 — NOx Adsorber Efficiency Below Threshold Bank 1
- P2001 — NOx Adsorber Efficiency Below Threshold Bank 2
Frequently asked questions
Why does P2189 appear only at idle and not at higher RPM?
At idle, the throttle plate is nearly closed, so any unmetered air (e.g. from a vacuum leak) represents a large fraction of total intake airflow and causes a significant lean swing. At higher RPMs and load, the throttle is more open and the same leak volume is proportionally much smaller relative to total airflow, so fuel trims may return to normal — making vacuum leaks the prime suspect for idle-only lean codes.
Which side of the engine is Bank 2?
Bank 2 is the cylinder bank that does not contain cylinder 1. On most V6/V8 engines with longitudinal mounting (rear-wheel-drive layout), this is typically the passenger side in North American market vehicles. Always verify with the manufacturer's service information for the specific vehicle, as front-wheel-drive transverse V6 engines define bank positions differently.
Can a dirty MAF sensor cause P2189?
Yes, but it will usually cause both Bank 1 and Bank 2 lean codes simultaneously, since the MAF measures total incoming airflow before the intake splits. A dirty MAF under-reports airflow, causing the ECM to under-fuel both banks. If only Bank 2 is lean, a bank-specific vacuum leak or injector issue is more likely than the MAF alone.
How do I tell if P2189 is caused by a vacuum leak versus a bad O2 sensor?
Check the Bank 2 fuel trims: if LTFT is genuinely elevated (ECM is adding fuel), the engine is truly lean and a vacuum leak or fuel delivery fault is the cause. If the upstream O2 sensor is stuck below 0.3 V but LTFT is only mildly elevated or normal, the sensor may be reading falsely lean. A smoke test is the fastest way to rule in or out a vacuum leak before condemning the sensor.
Disabling P2189 in software
RaceTune can permanently disable P2189 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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