Poor fuel economy: Codes, Causes & Fixes

When mpg drops without a change in driving habits, typical culprits are a thermostat stuck open (P0128), a slow oxygen sensor, a coolant temperature sensor reading too cold, dragging brakes or simply low tire pressure. Several of these set no code at first, so compare your recent fill-ups and check tire pressure against the placard on the driver's door jamb. A stored code, if present, narrows the search considerably.

146Codes

2.4Average severity

2%High or critical

17%DIY-friendly

Severity distribution

Most viewed

P0420 Powertrain The bank 1 catalytic converter no longer stores enough oxygen to clean the exhaust properly. Low P0101 Powertrain The MAF sensor's airflow reading doesn't match what the engine computer expects… Low P0113 Powertrain The intake air temperature signal is stuck at maximum voltage, which usually means an open… Low P0135 Powertrain The heater inside the upstream oxygen sensor on bank 1 isn't working or its circuit is faulty. Low 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

All codes (146)

All Severity 3+ Severity 4+

P2188 Powertrain At idle, bank 1 gets extra fuel from somewhere the PCM doesn't control; above idle the mixture… Moderate P2195 Powertrain The front O2 or air-fuel sensor keeps reading lean while the rear sensor shows the engine isn't… Low P2197 Powertrain The upstream sensor on the bank without cylinder 1 reads lean while the other checks say that… Low P2198 Powertrain Bank 2's front sensor keeps claiming rich, so the PCM leans out that side of the engine more… Low P219A Powertrain One cylinder on bank 1 runs clearly richer or leaner than the others, even if the average looks… Moderate P2201 Powertrain The upstream NOx sensor still communicates, but its readings don't match what the ECM expects. Moderate P2226 Powertrain The engine computer can't get a valid reading from its barometric (ambient air pressure) sensor. Low P2227 Powertrain The baro sensor's reading doesn't agree with the other pressure sensors or with what the engine… Low P2237 Powertrain The wire that carries the pump current of the upstream wideband (air-fuel) sensor is open. Low P2238 Powertrain The pump-current wire of the upstream air-fuel sensor is being pulled low, usually by a short… Low P2243 Powertrain The ECM lost the reference-cell signal of the upstream wideband sensor, as if the wire were… Low P2263 Powertrain Boost isn't tracking its target closely enough: too low, too high or too slow to respond. Moderate P2279 Powertrain Air is getting into the engine after the mass airflow sensor, so the computer isn't measuring… Low P242F Powertrain The DPF is filling up with ash that regeneration can't burn off; it needs cleaning… Moderate P244B Powertrain The DPF is resisting exhaust flow far more than it should – usually a filter packed with soot… Moderate P2453 Powertrain The DPF pressure reading doesn't make sense for the exhaust flow – most often a hose or tube… Moderate P2458 Powertrain A DPF regen took too long or never finished, usually from short trips or exhaust that won't get… Moderate P2459 Powertrain The DPF is regenerating far more often than it should for the miles driven. Moderate P2463 Powertrain The DPF is loaded with more soot than the ECU allows; it needs a regen or cleaning before it… High P2647 Powertrain The VTEC oil pressure switch reports the high-lift rockers engaged while the engine computer… Moderate P3000 Toyota · Powertrain Toyota's hybrid computer flags a fault in the high-voltage battery system and limits hybrid… High P3081 Volkswagen · Powertrain The engine didn't warm up as fast as the ECU expected – most often a thermostat stuck open. Low P3190 Toyota · Powertrain The hybrid computer asked the gas engine for power and got less than expected. Moderate P3400 Powertrain Cylinder shut-off on bank 1 (AFM, DFM, MDS or VCM) isn't doing what the engine computer… Moderate P3401 Powertrain The engine computer sees a broken or open circuit to the solenoid that shuts off cylinder 1. Low P3497 Honda · Powertrain Honda's Variable Cylinder Management on the front cylinder bank didn't behave the way the PCM… Moderate

Symptoms

Check engine light on 558 Loss of power 214 Rough idle 184 Limp mode 165 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