Fuel trim (air-fuel mixture) codes

How these codes differ

• P0171 and P0174 both mean too lean (too much air or too little fuel) and the computer has reached its correction limit; P0171 is bank 1 and P0174 is bank 2. P0172 and P0175 are the rich versions (too much fuel or too little air) for bank 1 and bank 2.
• On a V6 or V8, whether one bank or both banks set the code is the main clue. Both banks lean at once makes it worth checking first what feeds the whole engine (a mis-reading airflow sensor, low fuel pressure, or a large vacuum leak in the common intake). One bank alone points to something on that side only, such as an intake gasket leak or an injector on that bank.
• A lean code is a symptom code, not a part code: it tells you the computer is correcting, not which part failed. Watching fuel trim at idle versus higher engine speed helps split the causes: a vacuum leak usually shows most at idle, while a fuel delivery problem usually shows most under load.

Codes that show up together

• P0171 with P0174: both banks lean; look for a cause common to the whole engine before any one-bank part.
• P0171 or P0174 with misfire codes: a mixture that is lean enough can misfire, so the lean condition is usually treated first.
• A lean code with an airflow sensor code (P0101 or P0102): the airflow reading feeds the fuel calculation, so the sensor fault is the more specific lead.

• P0171 and P0174 together: both banks running lean

Fuel trim (air-fuel mixture) codes

• P0171

  Engine computer has reached its limit adding fuel on bank 1: the mixture is lean

  Severity: Monitor

• P0174

  Engine computer has reached its limit adding fuel on bank 2: the mixture is lean

  Severity: Monitor

• P0172

  Engine computer has reached its limit removing fuel on bank 1: the mixture is rich

  Severity: Monitor

• P0175

  Engine computer has reached its limit removing fuel on bank 2: the mixture is rich

  Severity: Monitor

• P219A

  The engine computer has detected that the cylinders on bank 1 are running at noticeably different air-fuel ratios from each other

  Severity: Inspect soon

• P2096

  The engine computer has had to add fuel based on the downstream oxygen sensor because the exhaust after the catalytic converter reads lean on bank 1

  Severity: Monitor

• P219B

  The engine computer has detected that one or more cylinders on bank 2 are running a noticeably different air-fuel mixture from the others

  Severity: Inspect soon

• P2097

  The engine computer has had to take fuel away based on the downstream oxygen sensor because the exhaust after the catalytic converter reads rich on bank 1

  Severity: Monitor

• P219F

  Cylinder 4 is running at a noticeably different air-fuel ratio from the other cylinders, so the computer flags an imbalance on that cylinder

  Severity: Inspect soon

• P04DB

  The computer believes the crankcase ventilation system has come apart, so crankcase pressure is not behaving as it should

  Severity: Monitor

• P2099

  The engine computer sees a rich mixture on bank 2 from the sensor behind the catalytic converter and the fuel trim cannot correct it

  Severity: Monitor

• P219C

  The engine computer has detected that cylinder 1 is running at a noticeably different air-fuel ratio from the other cylinders

  Severity: Severity not verified

• P219D

  The engine computer has detected that cylinder 2 is running at a noticeably different air-fuel ratio from the other cylinders

  Severity: Severity not verified

• P0170

  Bank 1 fuel trim has gone out of range

  Severity: Monitor

• P1174

  On Chevrolet and Saturn, fuel trim shows a cylinder balance problem on bank 1

  Severity: Severity not verified

• P219E

  Cylinder 3 air-fuel ratio imbalance

  Severity: Severity not verified

• P2191

  Fuel mixture too lean at higher load (bank 1)

  Severity: Severity not verified

• P04FB

  Crankcase ventilation pressure sensor reading does not make sense

  Severity: Severity not verified

• P0173

  Fuel trim at its limit on bank 2

  Severity: Severity not verified

• P2177

  Engine computer sees a lean mixture on bank 1 off idle (while cruising or at light throttle): too much air for the fuel being delivered

  Severity: Monitor

• P2187

  Engine computer sees a lean mixture on bank 1 at idle: too much air for the fuel being delivered

  Severity: Monitor

• P2C90

  Crankcase ventilation hose A disconnected

  Severity: Inspect soon