P0171 Engine is running lean (bank 1)

Check this first

Look for a split intake hose or a loose oil cap or dipstick – unmetered air is the number one cause.

Lean running makes an engine hot. Fix it before it turns into misfires or a cooked catalytic converter.

What usually fixes it

In the order a good shop checks them. Typical total: $62–$844 at $130 an hour.

1. Vacuum or intake leak

   $171–$444

   Cracked hoses and leaking gaskets let in air the computer didn't measure.

2. Clean or replace the air flow sensor

   $62–$401

   A dirty sensor under-reads air, so the computer adds too little fuel.

3. Dirty fuel injectors or weak fuel pressure

   $196–$844

   Not enough fuel gives the same lean reading.

4. Upstream oxygen sensor

   $128–$350

   An aging sensor reports lean when the mixture is fine.

Code titles are the generic SAE definitions. Prices are typical US parts and labor for a common car or light truck – a guide for judging a quote, not a quote. Always confirm a repair against the vehicle in front of you.