What P0174 means

Your engine computer is seeing too much air and not enough fuel on bank 2 — the side of the engine opposite cylinder number one — and it's maxed out how much extra fuel it can add to compensate.

Common symptoms

• Check engine light on (sometimes that's the only sign)
• Rough or surging idle, usually worst when the engine is cold
• Hesitation or stumble when you hit the gas
• Occasional misfire or a light spark knock under load
• Fuel economy slipping a little
• Plenty of these cars drive almost normal — don't let that fool you into ignoring it

One thing worth knowing up front: if you've got P0174 together with P0171 (same problem, bank 1), the cause is almost always something both banks share — the MAF sensor, fuel pressure, or a big vacuum leak. P0174 by itself points at something on the bank 2 side specifically.

Most likely causes, in the order a mechanic actually checks them

1. Vacuum leaks. Cracked or disconnected vacuum hoses, brittle PCV lines and elbows, leaking intake gaskets. This is the number-one cause of lean codes, period — especially on engines past 100k miles where the rubber and plastic have dried out.
2. Dirty MAF sensor. If the MAF underreports the air coming in, the computer underdelivers fuel. A can of MAF cleaner costs about ten bucks.
3. Intake manifold gasket leaking on the bank 2 side. The classic reason for P0174 showing up alone, without P0171.
4. Low fuel pressure. Tired fuel pump, clogged filter, weak regulator. Usually sets both bank codes, but not always.
5. Dirty or restricted fuel injectors.
6. Exhaust leak before the bank 2 upstream O2 sensor. A cracked manifold or broken studs let outside air hit the sensor, and it reads lean.
7. A lazy O2 sensor. Dead last on purpose. People throw O2 sensors at lean codes all the time, and most of the time the sensor was just telling the truth.

What I check first in the shop

First thing I do is plug in the scanner and watch the fuel trims live — at idle, then holding around 2,500 rpm. If the trims are way high at idle but come back toward normal when I rev it, that's a vacuum leak talking, and the smoke machine usually finds it in ten minutes. If it's lean everywhere, I'm looking at the MAF reading and fuel pressure before I touch a single part. The freeze-frame data saved with the code — rpm, load, coolant temp — usually tells me which way to go before I even open the hood.

Can you keep driving?

Short term, usually yes. This isn't a tow-it-now code. But a lean mixture burns hotter than it should, and if you ignore it for months you're risking misfires, damage to the catalytic converter, and real engine heat on long highway pulls or while towing. My honest advice: drive gently if you have to, skip the trailer and the heavy right foot, and get it diagnosed within a week or two — sooner if it starts misfiring.

Rough cost, DIY vs shop

Broad ballparks only — year, make, and engine move these numbers a lot:

• Vacuum hose or PCV line: a few dollars to $30 in parts DIY; $100–$200 at a shop including diagnosis
• MAF cleaning: about $10 DIY; often bundled into a shop diag
• Intake manifold gasket: $30–$80 in parts, but it's a labor job — a few hundred dollars at a shop depending on the engine
• Fuel pump: roughly $100–$400 in parts DIY; $400–$900+ at a shop depending on the vehicle
• Most shops charge $100–$180 for diagnosis, and many apply it toward the repair

Scan it yourself before the shop visit

You don't need my shop to find out why your light is on. Auto AI Diagnostics reads codes for free with any cheap Bluetooth OBD2 adapter — a $30 ELM327 or vLinker works fine. AXEL, the AI in the app, pulls the freeze-frame data from YOUR car — the rpm, engine load, and fuel trims at the exact moment P0174 set — and explains what it actually points to in plain English or Spanish. Walk into any shop already knowing what's likely wrong, or fix the cheap stuff yourself first.