Rough idle: Codes, Causes & Fixes

A shaky or uneven idle is most often caused by a vacuum leak, a misfire from worn spark plugs or a weak ignition coil, a dirty throttle body or an EGR valve stuck partly open. Related codes include P0300–P0312, lean codes like P0171 and idle control codes like P0505. Listen for hissing around intake hoses with the engine running and note whether the idle is worse when the engine is cold or warm.

184Codes

2.7Average severity

10%High or critical

19%DIY-friendly

Severity distribution

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P0174 Powertrain The cylinder bank without cylinder 1 is running lean even with the maximum fuel correction… Moderate P0101 Powertrain The MAF sensor's airflow reading doesn't match what the engine computer expects… Low P0171 Powertrain The engine computer is adding the maximum extra fuel it's allowed on bank 1 and still sees… Moderate P0172 Powertrain The engine computer is pulling as much fuel as it's allowed on bank 1 and the mixture is still… Moderate P0300 Powertrain The engine computer sees misfires that jump between cylinders or hit several cylinders at once. High P0302 Powertrain Cylinder 2 keeps misfiring. On inline engines it's a middle cylinder, so rule out coolant… Moderate

All codes (184)

All Severity 3+ Severity 4+

P3497 Honda · Powertrain Honda's Variable Cylinder Management on the front cylinder bank didn't behave the way the PCM… Moderate U0105 Network The PCM lost contact with the separate module that fires the injectors, common on diesel… High U0107 Network The engine computer can't reach the module that drives the electronic throttle, so power is cut… High U0115 Network Modules lost contact with the second engine computer on vehicles that use two of them. High

Symptoms

Check engine light on 558 Loss of power 214 Limp mode 165 Poor fuel economy 146 Engine stalling 133 Car won't start 123 Harsh or slipping shifts 80 ABS / traction light on 65 Fuel smell 34 Check engine light flashing 22