The one code that explains half your other codes

If you have pulled P0171 alongside two or three other codes, read this one before you start pricing parts for all of them. There is a decent chance one fix clears most of the list.

What the code actually means

P0171 reads "System Too Lean, Bank 1." The engine's oxygen sensor is reporting more oxygen in the exhaust than the fuel trim tables expect for the amount of fuel being injected — in plain terms, there is too much air relative to fuel. The ECU tries to correct for it by adding fuel (positive fuel trim), and the code sets when that correction runs out of room.

The cause is almost always air getting into the engine somewhere it should not — a vacuum leak in a hose or gasket, a dirty or failing mass airflow sensor under-reporting the air it sees, a stuck-open PCV valve, or in rarer cases a weak fuel pump that cannot deliver enough fuel to keep up even when the air side is normal.

Why this is the hub, not a spoke

Every one of P0171's top companions is a code this blog has already covered on its own:

A lean mixture is a plausible upstream cause for all three kinds of fault above it. A cylinder starved of fuel relative to air is more prone to misfire, which is why P0300 and P0301 and P0303 — the random and cylinder-specific misfire codes covered in the misfire post — turn up in roughly one in six to one in seven P0171 scans. A catalyst working overtime to burn off a persistently lean mixture is the kind of stress that shortens its life, which is exactly the mechanism explained in the catalyst-code post — and P0420 shows up almost as often. P0130, an oxygen sensor circuit fault, is the sensor doing the reporting in the first place, and a failing sensor can both cause and be caused by the same lean condition.

The practical order this suggests

Fix the vacuum leak or the MAF fault, and you may not need to touch the misfire, the catalyst or the oxygen sensor at all. That is the reason this is worth treating as the hub post for this whole cluster rather than chasing each companion code as its own repair: P0171 is frequently the cheapest fix in the group and the one most likely to make the others go away on their own, in the same way a vacuum leak is one of the standard causes listed for a random misfire in the first place.

It does not always work out that cleanly — a genuinely dead oxygen sensor or a converter that has already been cooked by months of lean running will not un-fail once the leak is patched. But checking the lean condition first, before replacing a sensor or a converter, is the order that costs the least money to get wrong.

P0172 is the mirror image

P0171's rich-mixture counterpart, P0172, is far rarer in our data — 3,097 scans against P0171's 8,695 — but it is worth knowing it exists, because a rich condition stresses the catalyst by a different mechanism and still shows up alongside P0420 at 27.12% of scans. If your fuel trims are running negative rather than positive, you are looking at the same family of problem from the other side: too much fuel rather than too little air.

What to do next

Start with a smoke test or a visual check of the vacuum lines and intake boot for cracks — this is the cheapest possible cause and the most common one. Check the MAF sensor for a dirty element the same way described in the P0113 companion post on this blog. Pull the fuel trim numbers if your scanner shows them: trims that are high at idle and settle down under load point at a vacuum leak specifically, rather than a fuel delivery problem. Only after the air side checks out clean does it make sense to test fuel pressure and the oxygen sensor itself.

Can you keep driving?

Yes, in the short term — a lean condition alone will not strand you. But every week spent lean is a week of extra stress on the catalyst and a bigger bill waiting behind it, so this is one worth chasing down while it is still a vacuum hose rather than a converter.