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 P0300 Powertrain The engine computer sees misfires that jump between cylinders or hit several cylinders at once. High 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 P0302 Powertrain Cylinder 2 keeps misfiring. On inline engines it's a middle cylinder, so rule out coolant… Moderate

All codes (18)

All Severity 3+ Severity 4+

P0016 Powertrain The bank 1 intake cam and the crankshaft are out of step, most often from a stretched or jumped… High P0018 Powertrain The bank 2 intake cam is out of step with the crankshaft on a V or boxer engine; chain wear… High P0200 Powertrain The ECU found a fault in the injector control circuits that it can't pin on one cylinder, often… High P0300 Powertrain The engine computer sees misfires that jump between cylinders or hit several cylinders at once. High P0320 Powertrain The computer lost the engine speed (rpm) signal. The engine can stall suddenly or refuse… High P0335 Powertrain The engine computer lost the crankshaft position signal. Expect stalling or a crank-no-start. High P0338 Powertrain The crank signal line sits at 5 or 12 volts and stops switching: open ground, open wire… High P06DD Powertrain The ECM commanded an oil pressure change and the measured pressure didn't respond – check… High P0742 Powertrain The converter lockup clutch stays applied when it should be released, which can stall… High P1295 Hyundai · Powertrain Hyundai's engine computer no longer trusts the electronic throttle and has cut power as… High P1340 Volkswagen · Powertrain The cam signal no longer lines up with the crank signal – often a jumped belt or stretched… High P2111 Powertrain The throttle plate stayed more open than commanded – a real safety concern until you know why. High P2112 Powertrain The PCM asked the throttle to open, but it stayed shut – the engine can't get air, so it bogs… High P2146 Powertrain The shared power feed for one group of injectors is open, so every injector in that group has… High P2149 Powertrain The shared supply to the second injector group dropped out, often an intermittent wiring fault… High 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