Reading a fault code: the letter and four digits
P0171 is not a random string but an address. We take it apart, sign by sign.
The first letter — whose fault it is
P — powertrain, B — body (airbags, locks, climate), C — chassis (brakes, ABS, steering), U — network between the units. More than half of the codes a regular driver sees start with P.
The first digit — generic or manufacturer
0 — generic, meaning the same on a Toyota and on a Ford. 1 — manufacturer specific: the meaning was defined by the carmaker and may be missing from a general directory. Digits 2 and 3 are rarer and mean the same split.
The second digit — which system
1 and 2 — fuel and air, 3 — ignition and misfires, 4 — emission control, 5 — idle, 6 — the control unit and its wiring, 7 and 8 — the gearbox.
The last two — what exactly
This is the number of the exact fault inside the system. It is what tells «lean mixture on bank 1» from «rich mixture on bank 2», and it is what you look up in the directory.
Check whether the code is stored or current
A current code is on right now, a stored one is left from an earlier trip, and a pending one is noted down and becomes real if it repeats. Chasing a single month-old stored code is usually not worth it.
Look at the freeze frame
When the code was stored, the car saved the RPM, speed, temperature and load. They show the conditions the trouble happens in: cold start, motorway or city traffic.