The 50 most common OBD2 codes mechanics see every day. Ranked by frequency. Likely cause, DIY fix, and parts cost for each.
| Code | Name & Most Likely Cause | DIY Fix Cost | Severity |
|---|---|---|---|
| P0420 | Catalyst Efficiency Below Threshold (Bank 1) Likely: downstream O2 sensor or aging catalytic converter. Test O2 voltage FIRST — a $50 sensor often fixes this without replacing the $800 cat. | $50-$2,400 | Med |
| P0171 | System Too Lean (Bank 1) Likely: vacuum leak, dirty MAF sensor, or failing fuel pump. Clean MAF with CRC sensor cleaner first ($8) before anything else. | $8-$600 | Med |
| P0300 | Random/Multiple Cylinder Misfire Likely: spark plugs (60% of cases) or ignition coils. Pull plugs and check gap — most modern cars need new plugs at 100k miles. | $30-$400 | High |
| P0301 | Cylinder 1 Misfire Single-cylinder misfire = isolated to plug or coil on that cylinder. Swap coil with cylinder 2 — if misfire moves, replace coil ($30-$80). | $30-$180 | High |
| P0455 | EVAP System Large Leak 90% of the time: loose or cracked gas cap. Tighten cap, drive 50 miles. If code returns, replace cap ($15). | $0-$80 | Low |
| P0440 | EVAP System Malfunction Gas cap, charcoal canister, or purge valve. Smoke test the EVAP system to pinpoint. | $15-$300 | Low |
| P0128 | Coolant Temp Below Thermostat Regulating Temp Almost always a stuck-open thermostat. $20 part, 1-hour DIY job on most cars. | $20-$280 | Med |
| P0430 | Catalyst Efficiency Below Threshold (Bank 2) Same as P0420 but on the other bank (V6/V8 engines). Test downstream O2 voltage before replacing cat. | $50-$2,400 | Med |
| P0174 | System Too Lean (Bank 2) Pair with P0171 — if both banks are lean, it's an intake leak or fuel pressure issue, NOT individual sensor failure. | $8-$600 | Med |
| P0302 | Cylinder 2 Misfire Swap test: move coil to another cylinder. If P0302 becomes P03XX matching the new cylinder, replace coil. | $30-$180 | High |
| P0303 | Cylinder 3 Misfire Same diagnosis pattern: spark plug, ignition coil, or rarely a stuck injector. | $30-$180 | High |
| P0304 | Cylinder 4 Misfire Same diagnosis pattern. If misfire follows the coil after swapping, it's the coil. | $30-$180 | High |
| P0401 | EGR Insufficient Flow Carbon-clogged EGR valve. Try cleaning before replacing — half the time it works. | $15-$450 | Low |
| P0442 | EVAP Small Leak Tiny leak — usually gas cap O-ring or hairline crack in EVAP line. Smoke test to find it. | $15-$200 | Low |
| P0507 | Idle Speed Higher Than Expected Vacuum leak or dirty throttle body. Clean throttle body first ($8 cleaner, 30-min DIY). | $8-$200 | Low |
| P0700 | Transmission Control System Malfunction Generic code — read the SECOND code stored (P07XX or P08XX) for the actual fault. Scan with a real reader. | Varies | Med |
| P0335 | Crankshaft Position Sensor Circuit Failed crank sensor — engine may stall or not start. $30-$90 part, usually accessible. | $30-$250 | High |
| P0340 | Camshaft Position Sensor Circuit Cam sensor failure or wiring issue. Test sensor with multimeter before replacing. | $25-$200 | Med |
| P0500 | Vehicle Speed Sensor Malfunction VSS failure — speedometer may flicker. $30-$80 sensor, located on transmission output. | $30-$180 | Med |
| P0411 | Secondary Air Injection System Air injection pump or one-way valves failing. Common on older German cars and Toyotas. | $80-$900 | Low |
| P0172 | System Too Rich (Bank 1) Opposite of P0171: too much fuel. Check fuel pressure regulator, MAF sensor, or stuck-open injector. | $30-$600 | Med |
| P0017 | Camshaft/Crankshaft Correlation Timing chain stretched or VVT solenoid stuck. On GM 3.6L V6, often a $400 chain replacement. | $80-$2,000 | High |
| P0011 | Intake Camshaft Position - Timing Over-Advanced VVT solenoid clogged with sludge. Replace solenoid AND change oil — usually caused by missed oil changes. | $60-$400 | Med |
| P0014 | Exhaust Camshaft Position - Timing Over-Advanced Exhaust-side VVT solenoid. Same fix as P0011 but on the exhaust cam. | $60-$400 | Med |
| P0102 | MAF Sensor Low Voltage Dirty or failed MAF. Clean with CRC MAF cleaner ($8) — fixes 70% of cases. | $8-$280 | Med |
| P0118 | Engine Coolant Temp Circuit High Coolant temp sensor failure. $15-30 part, easy DIY swap. | $15-$120 | Med |
| P0131 | O2 Sensor Low Voltage (Bank 1 Sensor 1) Upstream O2 sensor likely failing or contaminated. ~$50 sensor. | $50-$250 | Med |
| P0156 | O2 Sensor Circuit (Bank 2 Sensor 2) Downstream O2 sensor on bank 2. Failed sensor or wiring damage. | $60-$400 | Med |
| P0190 | Fuel Rail Pressure Sensor Circuit Fuel pressure sensor failed. Common on diesels (Cummins, Duramax, Powerstroke). | $80-$450 | High |
| P0524 | Engine Oil Pressure Too Low Low oil level OR failed oil pressure sensor. CHECK OIL FIRST. If level is fine, swap sensor before assuming engine failure. | $20-$2,000 | High |
| Code | Name & Most Likely Cause | DIY Fix Cost | Severity |
|---|---|---|---|
| C0034 | Front Right Wheel Speed Sensor Circuit Wheel speed sensor failed or wiring damaged. Affects ABS/traction control. $30-$80 sensor. | $30-$220 | Med |
| C0020 | ABS Pump Motor Circuit ABS module failure. Often a wiring connector issue, not the pump itself. Inspect connectors first. | $0-$1,200 | Med |
| U0100 | Lost Communication With ECM/PCM CAN bus issue — usually a corroded ground or damaged wiring. Check ground straps before swapping modules. | $0-$1,500 | High |
| U0102 | Lost Communication With Transfer Case Control Module 4WD module communication failure. Check fuses and ground connections at transfer case. | $0-$900 | Med |
| U0073 | Control Module Communication Bus Off Network is silent — usually a short on the CAN bus. Diagnostic challenge, may need a shop scanner. | $0-$1,200 | Med |
| B1342 | ECU is Defective Body control module fault. Sometimes resolvable with a battery disconnect reset. | $0-$1,500 | Med |