OBD-II Code · Fuel & Air
P0174
System Too Lean (Bank 2)
Severity
medium
Worth booking. It costs fuel, emissions or comfort until it is fixed.
Can you drive it
Only if running normally
A short trip to a shop may be reasonable if the light is steady and the car runs normally. Stop safely for a flashing light, severe shaking, overheating, loss of power, or another urgent warning. The code alone cannot confirm safety.
Indicative cost (USD)
$50–$600
Varies by vehicle, diagnosis, parts, and local labor. This is not a repair quote.
Smoke test first
Bank 2 running lean. Same causes as P0171 but on bank 2 (V-engines only).
How a shop works this
Lean means the ECU is already adding fuel and the sensor still reports lean. So the question is not whether it is lean — it is whether air is getting in unmetered or fuel is not getting there. One comparison separates them.
Read short-term and long-term fuel trim on the affected bank, warm, at idle.
Positive means the ECU is adding fuel, negative means it is taking fuel away. Read short-term and long-term together rather than either on its own. As a rule of thumb rather than a specification, correction beyond roughly plus or minus 10% is worth chasing. This code is specific to bank 2. If bank 1 is not also lean, the fault is mechanical and local to bank 2 — its intake runners and gaskets, its PCV or breather routing, or an exhaust leak ahead of its upstream sensor.
Compare those trims at idle against roughly 2,000 to 2,500 rpm.
A leak is a fixed amount of air. At idle it is a large fraction of what the engine is breathing; at 2,500 rpm it is a small one. That is why the number moves.
- Strongly positive at idle, much closer to zero at 2,000 to 2,500 rpmUnmetered air — a vacuum or intake leak. At idle the leak is a large share of everything the engine is breathing; at higher rpm it is swamped by it, which is why the number improves. Go find the leak.
- Positive at idle and still positive as the rpm comes upFuelling or measurement, not a leak. Fuel pressure at the rail, a MAF reading low for the engine, or an exhaust leak ahead of the upstream oxygen or air-fuel sensor letting fresh air past it.
- NegativeThe ECU is pulling fuel out, so the engine is running rich rather than lean. Look at a leaking injector, a purge valve stuck open, or fuel pressure that is too high.
Find the leak.
Start with what you can see: the intake ducting and its boot, the vacuum hoses, the PCV system, and anything obviously cracked or disconnected. A smoke test is the reliable way to locate one, and most shops will do it quickly. Do not spray a flammable solvent around a running engine to hunt a leak — it is an old trick and it does start fires.
Still lean with no leak found: fuel pressure at the rail, then the MAF.
Most port-injection systems sit around 35 to 55 psi at idle; below spec points at the pump, the filter or the regulator. A rough sanity check at warm idle is around 1 gram per second per litre of displacement — near 2 on a 2.0-litre four, near 5 on a 5.0-litre V8. Displacement and conditions move it a lot, so treat it as a smell test rather than a spec.
Before condemning a sensor, check what is ahead of it in the exhaust.
An exhaust leak upstream of the oxygen or air-fuel sensor pulls in outside air, the sensor reads lean, and the ECU adds fuel the engine never needed. In the data it looks exactly like a vacuum leak, and it costs a lot less to find than a new sensor costs to fit.
If the code comes back after a repair, the underlying fault is still there. That is the point to bring in a scanner with live data and freeze-frame, or a shop that has one.
Common symptoms
- Check engine light
- Rough idle
- Hesitation on acceleration
Likely causes
- Vacuum or intake leak on bank 2Most common
- Exhaust leak ahead of the bank 2 upstream O2 sensor
- MAF sensor reading low
- Weak fuel delivery to bank 2
- Failing bank 2 O2 sensor
Read the full diagnostic procedureP0174 is the bank-2 mirror of P0171 — the ECU has run long-term fuel trim on bank 2 past +25% trying to add fuel and the upstream O2 sensor still rea…
P0174 is the bank-2 mirror of P0171 — the ECU has run long-term fuel trim on bank 2 past +25% trying to add fuel and the upstream O2 sensor still reads lean.
Because the code is bank-specific, the diagnostic narrows fast
anything that affects both banks simultaneously (low fuel pressure, dirty MAF, failed pre-cat O2 reference) will throw P0171 AND P0174 together; if only P0174 is set, the fault is mechanically isolated to bank 2's intake or exhaust path. Cheapest-first order on a shop floor: smoke-test the intake with the smoke wand fed into the brake booster line or PCV port and watch the bank-2 manifold runners, intake gaskets, and the bank-2 PCV/breather hose route — about 70% of single-bank lean codes are vacuum leaks on the runner-gasket or a cracked rubber elbow specific to that bank. Then check for an exhaust leak between the bank-2 manifold and the upstream O2 sensor (a leak there pulls in fresh atmospheric air, makes the sensor read lean, ECU dumps in fuel that never gets used). Finally, swap bank-1 and bank-2 upstream O2 sensors and see if the code follows the sensor — if it does, it's a bad O2; if it stays on bank 2, the problem is mechanical. STFT bouncing >+15% under steady-state cruise is the live-data signature.
Vehicle-specific patterns2004-2010 Ford F-150 / Expedition / Navigator with the 5.4L 3-valve Triton throws P0174 alone from a cracked composite intake manifold on the passeng…
- 2004-2010Ford F-150 / Expedition / Navigator with the 5.4L 3-valve Triton throws P0174 alone from a cracked composite intake manifold on the passenger (bank 2) side near the EGR port — Ford issued multiple TSBs and the upgraded manifold with aluminum coolant crossover is the durable fix ($350-$500 part, $700-$1,100 installed).
- 2005-2010Chrysler 300 / Charger / Magnum with the 2.7L or 3.5L V6 commonly throws P0174 from cracked intake-manifold plenum gaskets on the rear (bank 2) bank.
- 2000-2007BMW E46 / E39 / X5 with the M54 inline-6 throws P0174 from disintegrating DISA valve O-rings and crankcase-vent (CCV) diaphragm failure that vents directly into the bank-2 intake path.
- 2006-2012GM trucks with the Vortec 5.3L throw P0174 from a failed manifold-absolute-pressure-style intake gasket on the bank-2 side around 100k miles. Repair cost typically $50-$600: a cracked vacuum hose is the low end, full intake-manifold replacement on the 5.4L Triton is the high end.
Look up another code
P0174 — frequently asked
What does a P0174 code mean?
Bank 2 running lean. Same causes as P0171 but on bank 2 (V-engines only).
What causes a P0174 code?
Common causes: Vacuum or intake leak on bank 2; Exhaust leak ahead of the bank 2 upstream O2 sensor; MAF sensor reading low; Weak fuel delivery to bank 2; Failing bank 2 O2 sensor.
What are the symptoms of a P0174 code?
Typical symptoms: Check engine light; Rough idle; Hesitation on acceleration.
Is it safe to drive with a P0174 code?
Only if running normally. A short trip to a shop may be reasonable if the light is steady and the car runs normally. Stop safely for a flashing light, severe shaking, overheating, loss of power, or another urgent warning. The code alone cannot confirm safety. Estimated repair: $50-$600 — smoke test first.
How do I diagnose a P0174 code?
Lean means the ECU is already adding fuel and the sensor still reports lean. So the question is not whether it is lean — it is whether air is getting in unmetered or fuel is not getting there. One comparison separates them. 1. Read short-term and long-term fuel trim on the affected bank, warm, at idle. 2. Compare those trims at idle against roughly 2,000 to 2,500 rpm. 3. Find the leak. 4. Still lean with no leak found: fuel pressure at the rail, then the MAF. 5. Before condemning a sensor, check what is ahead of it in the exhaust.
More free tools