Car Problems by Symptom

Don't know the fault code? Start with what the car is doing. Each guide lists the OBD-II codes most likely to be stored, the common causes, and a safe order to diagnose — 13 symptoms covered.

Rough Idle or Shaking at a Stop

9 codes to check

The engine idles unevenly, vibrates, or nearly dies when you sit at a light. The ECM usually detects this as unstable RPM and may store a misfire or fuel-trim code.

OBD-II codes to check

P0300 Random/Multiple Cylinder Misfire Detected High P0301 Cylinder 1 Misfire Detected Med P0302 Cylinder 2 Misfire Detected Med P0303 Cylinder 3 Misfire Detected Med P0304 Cylinder 4 Misfire Detected Med P0171 System Too Lean (Bank 1) Med P0174 System Too Lean (Bank 2) Med P0506 Idle Air Control System RPM Lower Than Expected Med P0507 Idle Air Control System RPM Higher Than Expected Med

Common causes

  • Vacuum leak (intake gaskets, PCV hoses)
  • Dirty or failing mass air flow (MAF) sensor
  • Worn spark plugs or failing ignition coils
  • Dirty idle air control valve / throttle body
  • Low fuel pressure or clogged injector

Diagnosis order

  1. Scan for stored codes and check freeze-frame data
  2. Spray-check intake gaskets and hoses for vacuum leaks
  3. Inspect spark plugs, coils and boots
  4. Clean the MAF sensor and throttle body with the correct cleaners
  5. Verify fuel pressure against spec

⚠ If the check engine light starts flashing, stop driving and get it diagnosed — a catalytic converter can be damaged in minutes.

Flashing Check Engine Light

5 codes to check

A flashing check engine light means a severe misfire is happening right now. Unburned fuel is entering the exhaust, which can destroy the catalytic converter.

OBD-II codes to check

P0300 Random/Multiple Cylinder Misfire Detected High P0301 Cylinder 1 Misfire Detected High P0302 Cylinder 2 Misfire Detected High P0303 Cylinder 3 Misfire Detected High P0304 Cylinder 4 Misfire Detected High

Common causes

  • Failing ignition coil or coil-on-plug
  • Worn or fouled spark plugs
  • Fuel injector stuck open or plugged
  • Low compression in one or more cylinders
  • Large vacuum leak after the MAF sensor

Diagnosis order

  1. Safely pull over and stop — do not keep driving
  2. Scan the code and note which cylinder(s) misfire
  3. Swap coil(s) and plugs between cylinders to isolate the fault
  4. Check injector pulse and fuel pressure
  5. Compression-test the misfiring cylinder if the above are fine

⚠ Flashing light = stop as soon as it is safe. Continued driving risks a $1,000+ catalytic converter replacement.

Hard Start or No Start

7 codes to check

The engine cranks but is slow to fire, or cranks without starting at all. Crank/cam sensor, fuel pressure and battery supply are the usual suspects.

OBD-II codes to check

P0335 Crankshaft Position Sensor 'A' Circuit High P0336 Crankshaft Position Sensor 'A' Circuit Range/Performance High P0340 Camshaft Position Sensor 'A' Circuit Med P0341 Camshaft Position Sensor 'A' Circuit Range/Performance Med P0087 Fuel Rail/System Pressure Too Low High P0230 Fuel Pump Primary Circuit Malfunction High P0627 Fuel Pump 'A' Control Circuit/Open High

Common causes

  • Failing crankshaft or camshaft position sensor
  • Low fuel pressure (weak pump, clogged filter, bad regulator)
  • Fuel pump relay or wiring fault
  • Weak battery or poor grounds
  • Crank no-start with no codes can also be immobilizer related

Diagnosis order

  1. Check battery voltage and charge state first
  2. Verify fuel pressure and that the pump primes on key-on
  3. Test crank/cam sensor output while cranking
  4. Check fuel pump relay and fuse
  5. Confirm the immobilizer is not preventing start

⚠ If the engine cranks but never fires, do not keep cranking in long bursts — fuel dilution and starter damage get expensive.

Engine Stalls While Driving

7 codes to check

The engine dies at stops or intermittently while driving, then usually restarts. Fuel delivery, air metering and idle control faults are the common causes.

OBD-II codes to check

P0101 Mass or Volume Air Flow Circuit Range/Performance Problem Med P0171 System Too Lean (Bank 1) Med P0174 System Too Lean (Bank 2) Med P0335 Crankshaft Position Sensor 'A' Circuit High P0340 Camshaft Position Sensor 'A' Circuit Med P0506 Idle Air Control System RPM Lower Than Expected Med P0507 Idle Air Control System RPM Higher Than Expected Med

Common causes

  • Dirty throttle body or failing idle air control valve
  • MAF sensor contamination or signal fault
  • Fuel pump / fuel pressure loss under load
  • Vacuum leak causing a lean condition
  • Crank or cam sensor signal drop-out

Diagnosis order

  1. Scan for codes and check freeze-frame (RPM, fuel trim) at the stall
  2. Clean the throttle body and IAC passages
  3. Test fuel pressure while driving with a gauge on the rail
  4. Inspect MAF signal with a scan tool
  5. Check wiring and connectors at the crank/cam sensors

⚠ Stalling in traffic is a safety issue — if the loss of power steering/brakes is a concern, have it towed rather than risk a stall mid-intersection.

Engine Overheating

5 codes to check

The temperature gauge climbs into the red or the warning light comes on. Coolant level, thermostat, fans and the coolant temperature sensor are checked first.

OBD-II codes to check

P0117 Engine Coolant Temperature Circuit Low Input Med P0118 Engine Coolant Temperature Circuit High Input Med P0128 Coolant Thermostat Below Regulating Temperature Low P0217 Engine Overheating Condition High P0480 Cooling Fan 1 Control Circuit Malfunction Med

Common causes

  • Low coolant or coolant leak
  • Stuck-closed thermostat
  • Cooling fan not running (relay, fuse, wiring)
  • Failing water pump or drive belt
  • Faulty coolant temperature sensor reading low

Diagnosis order

  1. If overheating, stop as soon as it is safe — never open the radiator while hot
  2. Check coolant level and look for leaks
  3. Verify both cooling fans run at operating temperature
  4. Test thermostat opening temperature
  5. Compare live coolant temperature with an infrared thermometer

⚠ P0217 means the ECM has detected an overheating condition. Continuing to drive can warp the head and destroy the engine.

Poor Fuel Economy

7 codes to check

Miles per gallon drop noticeably with no obvious cause. Fuel trims, oxygen sensors, air metering and a stuck-open thermostat are the most common culprits.

OBD-II codes to check

P0101 Mass or Volume Air Flow Circuit Range/Performance Problem Med P0131 O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1) Med P0133 O2 Sensor Circuit Slow Response (Bank 1 Sensor 1) Med P0171 System Too Lean (Bank 1) Med P0174 System Too Lean (Bank 2) Med P0420 Catalyst System Efficiency Below Threshold (Bank 1) Med P0128 Coolant Thermostat Below Regulating Temperature Low

Common causes

  • Aging or contaminated oxygen sensors
  • Dirty MAF sensor skewing air readings
  • Vacuum leak causing rich/lean fuel trims
  • Thermostat stuck open — engine never reaches operating temperature
  • Underinflated tires or dragging brakes (no code needed)

Diagnosis order

  1. Read long-term fuel trims — anything over ±10% points at a mechanical fault
  2. Test oxygen sensor response with a scan tool
  3. Clean the MAF sensor
  4. Verify coolant reaches thermostat regulation temperature
  5. Check tire pressure and brake drag

⚠ A slow O2 sensor or stuck thermostat quietly wastes fuel for months — fixing these is usually the cheapest mpg win.

Burning Oil / Blue Smoke

5 codes to check

Blue-gray smoke from the exhaust, a burning smell, or the oil level dropping between changes. Oil consumption is often mechanical and may store no code at all.

OBD-II codes to check

P0521 Engine Oil Pressure Sensor/Switch Range/Performance Med P0522 Engine Oil Pressure Sensor/Switch Circuit Low Med P0010 Camshaft Position Actuator Circuit/Open (Bank 1) Med P0011 Camshaft Position Timing Over-Advanced (Bank 1) Med P0300 Random/Multiple Cylinder Misfire Detected High

Common causes

  • Worn valve stem seals or guides
  • Worn piston rings or cylinder walls
  • Clogged PCV system
  • Turbocharger seal leakage (on turbo engines)
  • Oil pressure issues affecting VVT actuators

Diagnosis order

  1. Check the PCV valve and hoses first — cheapest common cause
  2. Watch for blue smoke on deceleration (valve seals) vs. acceleration (rings)
  3. Run a compression and leak-down test
  4. Verify oil pressure at operating temperature
  5. If oil pressure codes are present, inspect the sensor wiring before condemning the engine

⚠ Smoke only on startup that clears is usually valve seals; constant blue smoke under load suggests rings — a proper leak-down test settles it.

Misfire or Loss of Power

9 codes to check

Hesitation under acceleration, jerking, or a noticeable drop in power. A misfire under load can flash the light and log a cylinder-specific code.

OBD-II codes to check

P0300 Random/Multiple Cylinder Misfire Detected High P0301 Cylinder 1 Misfire Detected High P0302 Cylinder 2 Misfire Detected High P0303 Cylinder 3 Misfire Detected High P0304 Cylinder 4 Misfire Detected High P0101 Mass or Volume Air Flow Circuit Range/Performance Problem Med P0171 System Too Lean (Bank 1) Med P0174 System Too Lean (Bank 2) Med P0121 Throttle/Pedal Position Sensor 'A' Circuit Range/Performance Med

Common causes

  • Worn spark plugs, failing coils or boot arcing
  • Fuel injector clogged or leaking
  • Low fuel pressure under load
  • MAF or throttle position sensor signal faults
  • Low compression from rings, valves or head gasket

Diagnosis order

  1. Identify the exact misfiring cylinder(s) from freeze-frame
  2. Swap coil and plug between cylinders to see if the misfire follows
  3. Check fuel pressure under load
  4. Compression-test the affected cylinder
  5. Clean the MAF and throttle body if no ignition fault is found

⚠ Load misfires that flash the light need prompt attention — unburned fuel in the exhaust kills catalytic converters.

Transmission Shift Problems

5 codes to check

Hard or delayed shifts, slipping, or the car goes into limp mode. The TCM reports transmission codes to the PCM, which illuminates the check engine light.

OBD-II codes to check

P0700 Transmission Control System (MIL Request) High P0730 Incorrect Gear Ratio High P0750 Shift Solenoid 'A' Malfunction Med P0755 Shift Solenoid 'B' Malfunction Med P0780 Shift Malfunction High

Common causes

  • Low or contaminated transmission fluid
  • Failed shift solenoids or valve body wear
  • Internal clutch/band wear (incorrect gear ratio)
  • Transmission control module or wiring fault
  • Torque converter issues

Diagnosis order

  1. Check fluid level, color and smell first — burnt fluid means internal wear
  2. Scan all codes including the TCM's own codes
  3. Test solenoid resistance and supply voltage
  4. Verify fluid pressure with a gauge if possible
  5. Have a transmission shop perform a line-pressure test before any rebuild

⚠ Never ignore P0730 (incorrect gear ratio) — driving on it can turn a solenoid repair into a full rebuild.

ABS or Brake Warning Light

4 codes to check

The ABS light (and sometimes traction control light) stays on. The ABS module reports its own C-codes; a wheel speed sensor is the most common cause.

OBD-II codes to check

C0035 Left Front Wheel Speed Sensor Circuit Med C0040 Right Front Wheel Speed Sensor Circuit Med C0045 Left Rear Wheel Speed Sensor Circuit Med C0050 Right Rear Wheel Speed Sensor Circuit Med

Common causes

  • Failing wheel speed sensor or damaged wiring
  • Cracked or rusted reluctor/tone ring
  • Excessive wheel bearing play
  • ABS module fault
  • Battery voltage issues corrupting module memory

Diagnosis order

  1. Scan for C-codes to identify the affected wheel
  2. Inspect the sensor wiring and connector for damage
  3. Check the tone ring for cracks, rust or missing teeth
  4. Measure sensor resistance and AC output while spinning the wheel
  5. Clear codes and test drive after repair

⚠ ABS light on does not mean brakes fail — but it means no ABS/TCS, so drive with extra care in rain or snow until fixed.

Module Communication / Electrical Faults

5 codes to check

Dash warning lights come on together, gauges drop out, or the scan tool cannot reach certain modules. These are U-codes — CAN bus communication failures.

OBD-II codes to check

U0001 High Speed CAN Communication Bus Med U0100 Lost Communication With ECM/PCM Med U0101 Lost Communication With TCM Med U0121 Lost Communication With ABS Control Module Med U0140 Lost Communication With Body Control Module Med

Common causes

  • Weak or failing battery / charging system
  • Poor grounds or corroded connectors
  • Damaged CAN bus wiring (chafed, shorted, open)
  • A failed module taking the bus down
  • Aftermarket accessories wired into the bus

Diagnosis order

  1. Test battery voltage and charging output first
  2. Clean battery terminals and grounds
  3. Check CAN bus resistance (60 ohm at the ends, 120 ohm at each module)
  4. Inspect connectors for corrosion and bent pins
  5. Disconnect modules one at a time to isolate a bus-killer

⚠ Multiple U-codes at once usually trace back to power/ground or bus wiring, not to many modules failing at the same time.

Fuel Smell or EVAP Codes

5 codes to check

A gas smell around the car, or an EVAP system leak code after a failed emissions test. The system is sealed; any leak sets a code.

OBD-II codes to check

P0440 Evaporative Emission Control System Malfunction Low P0442 Evaporative Emission Control System Leak Detected (Small Leak) Low P0455 Evaporative Emission Control System Leak Detected (Gross Leak/No Flow) Med P0456 Evaporative Emission System Leak Detected (Very Small Leak) Low P0446 Evaporative Emission Control System Vent Control Circuit Malfunction Low

Common causes

  • Loose, missing or cracked fuel cap (most common)
  • Cracked or disconnected EVAP hoses
  • Failing purge valve stuck open or closed
  • Failing vent valve or charcoal canister
  • Damaged fuel filler neck

Diagnosis order

  1. Tighten or replace the gas cap and clear the code first
  2. Smoke-test the EVAP system to find the leak
  3. Check the purge valve operation with a scan tool
  4. Test the vent valve and canister
  5. Repair and re-run the emissions monitor drive cycle

⚠ P0455 (gross leak) usually means a big opening — check the cap and filler neck before replacing expensive components.

Battery / Charging System Warning

3 codes to check

The battery light is on, electronics act erratically, or the voltage reads wrong. Charging system codes come from the PCM watching system voltage.

OBD-II codes to check

P0562 System Voltage Low Med P0563 System Voltage High Med P0620 Generator Control Circuit Malfunction Med

Common causes

  • Weak or failing battery
  • Failing alternator or voltage regulator
  • Corroded battery terminals or loose grounds
  • Frayed charging circuit wiring
  • Excessive parasitic draw

Diagnosis order

  1. Test battery state of charge and health
  2. Measure charging voltage at idle and under load (13.5–14.8V typical)
  3. Check alternator output at the battery posts, not the alternator
  4. Inspect battery cables and grounds for resistance
  5. Test for parasitic draw if the battery drains overnight

⚠ Voltage below ~12V while running with the battery light on means the alternator is not charging — you will not go far.

Not sure where to start?

If your scanner already shows a code, look it up directly or browse by prefix (P powertrain, B body, C chassis, U network). For a specific make, browse codes by brand. Written guides live in the repair library.