Fault Code Lookup — Decode Any OBD Code
Fault Code Lookup
The fault code lookup our team uses on the bench: every common obd fault codes family explained — what the check engine code p0420 means, why P0300 and P0171 keep showing up, and which part to check first. Whether you need the p0420 code meaning for a failed catalyst or the obd fault codes behind a rough idle, start here, then ask us with your VIN.
How OBD Codes Work
Every diagnostic trouble code is a five-character code: one letter for the system, one digit for generic or manufacturer-specific, one digit for the subsystem, and two digits for the exact fault. That is how to read fault codes on any obd codes list — no manual needed once you know the layout.
| Position | Character | What it means | Example |
|---|---|---|---|
| 1st — letter | P / B / C / U | System: P = powertrain (engine, transmission, fuel, emissions), B = body (airbags, climate), C = chassis (ABS, steering, brakes), U = network (module communication) | P0420 |
| 2nd — digit | 0 or 1 | 0 = generic SAE code (same across all cars); 1 = manufacturer-specific code (unique to your brand) | P0420 |
| 3rd — digit | 0-8 | Subsystem: 1 = fuel and air metering, 2 = injector circuit, 3 = ignition/misfire, 4 = auxiliary emissions, 5 = vehicle speed and idle, 7-8 = transmission | P0420 → emissions |
| 4th-5th — digits | 00-99 | The specific fault within that subsystem — the part of the diagnosis you actually look up | P0420 → catalyst efficiency |
The obd ii standard has been mandatory on every car sold in the US since 1996 — so the same generic codes work across almost every vehicle on the road. Manufacturer codes (second digit 1, like P1301) are brand-specific and usually need dealer-level tools; send us the full code plus your VIN and we will interpret it for you.
Most Common P Codes, Explained
The P codes our diagnostic engineers see every day — the p0420 code, p0300 code and p0171 code lead the list, and each one points to a specific system. Find your code, check the causes, and order the right part.
| Code | Meaning | Likely causes | First check |
|---|---|---|---|
| P0420 code | Catalyst system efficiency below threshold (Bank 1) | Failing catalytic converter, faulty O2 sensors, exhaust leak, engine running rich | O2 sensors and exhaust before the converter — a p0420 fix is often a sensor, not a catalyst. See O2 sensor codes. |
| P0300 code | Random / multiple cylinder misfire | Spark plugs, ignition coils, injectors, vacuum leak, low compression | Spark plugs and coils first — a p0300 misfire across cylinders usually means tune-up parts. Read freeze frame to see conditions. |
| P0171 code | System too lean (Bank 1) | Vacuum leak, dirty MAF sensor, weak fuel pump, clogged fuel filter | Listen for hissing (vacuum), clean the MAF, check fuel pressure — the p0171 fix depends on which cause the fuel trims show. |
| P0172 code | System too rich (Bank 1) | Dirty or faulty MAF, faulty O2 sensor, leaking injector, high fuel pressure | Check MAF readings and fuel trims — a p0172 system too rich code often trails a failing O2 sensor or MAF. |
| P0128 code | Coolant thermostat below regulating temperature | Stuck-open thermostat, low coolant, faulty coolant temp sensor | Replace the thermostat and verify coolant level — a cheap repair that stops the code returning. |
| P0455 code | EVAP system gross leak detected | Loose or missing gas cap, cracked EVAP hose, failed purge valve | Tighten or replace the gas cap first — it fixes a surprising share of EVAP codes. |
| P0442 code | EVAP system small leak detected | Loose gas cap, cracked hose, faulty purge valve or leak detection pump | Same gas-cap-first rule; a smoke test finds the rest. |
| P0340 code | Camshaft position sensor circuit malfunction | Faulty sensor, wiring/connector, timing chain stretch | Inspect the sensor connector and wiring, then the sensor itself. |
| P0401 code | EGR system insufficient flow | Carbon-clogged EGR valve, blocked EGR passages, vacuum line | Clean the EGR valve and passages — carbon buildup is the usual culprit. |
| P0335 code | Crankshaft position sensor circuit malfunction | Faulty sensor, damaged wiring, tone ring damage | Check the sensor connector, gap, and the tone ring for damage. |
| P0500 code | Vehicle speed sensor circuit malfunction | Faulty VSS, wiring, damaged tone ring | Check the sensor and its wiring at the transmission or wheel hub. |
| P0430 code | Catalyst system efficiency below threshold (Bank 2) | Failing catalytic converter, faulty O2 sensors, exhaust leak (same as P0420, other bank) | Check O2 sensors and exhaust on Bank 2 — the p0430 catalyst system efficiency below threshold bank 2 test mirrors P0420. |
| P0174 code | System too lean (Bank 2) | Vacuum leak, dirty MAF, weak fuel pump, clogged filter (Bank 2) | Same diagnostic path as P0171, on the opposite bank — the p0174 system too lean bank 2 code trails the same causes. |
| P0301 code | Cylinder 1 misfire detected | Spark plug, ignition coil, injector, low compression on cylinder 1 | Swap coil and plug with another cylinder — if the misfire moves, the part is bad. A p0301 cylinder 1 misfire is the single-cylinder cousin of P0300. |
| P0700 code | Transmission control system malfunction | Internal transmission fault, TCM fault, wiring | The p0700 transmission control system code is a general flag — read the accompanying transmission codes for the specific fault. |
| P0456 code | EVAP system very small leak detected | Loose gas cap, pinhole in a hose, faulty purge valve | P0456 is the smallest EVAP leak threshold — start at the gas cap, then smoke test. |
| P0440 code | EVAP system malfunction | Purge valve, vent valve, hoses, gas cap | P0440 is the general EVAP flag; read P0442/P0455/P0456 for the specific leak size. |
Reading a p0420 bank 1 code with P0300 and P0171 together is common — one root cause like a vacuum leak or exhaust leak can set multiple codes, and the p0420 catalytic converter performance is often a downstream effect, not the original fault. Diagnose in order: leaks first, sensors second, catalysts last. If your code is not on this list, send it to our specialists with your VIN and we will look it up for you.
O2 Sensor Codes (P0130-P0135 Family)
Oxygen sensor codes are among the most common OBD codes — and the most misdiagnosed. The p0135 code means the O2 sensor heater circuit is malfunctioning; the oxygen sensor code list below covers the family our team wires most, because oxygen sensors are our specialty.
Heater circuit
P0135 — O2 Sensor Heater Circuit
The p0135 code means the heater circuit in the upstream O2 sensor is open or shorted — the sensor cannot warm up fast enough for closed-loop operation. The p0135 heater circuit check: measure heater resistance and verify the fuse; if the heater element is open, replace the sensor.
Upstream
P0130-P0134 — Upstream O2 Circuit Range
P0130 and its siblings mean the upstream sensor signal is out of range, slow or missing. Causes: faulty sensor, wiring, or exhaust leaks ahead of the sensor. A p0135 o2 sensor replacement is the fix when the heater or signal tests bad — see upstream vs downstream O2 comparison.
OE supply
Oxygen Sensor Code Diagnosis — We Supply the Part
Send us the oxygen sensor code plus your VIN and we will confirm the correct upstream or downstream sensor for your exact vehicle. We supply OE-grade oxygen sensors for VW, Audi, Seat, Skoda and more — matched to the OE number on your original part, not a guess.
Don't replace the catalytic converter on an O2 sensor code. A p0420 code is often caused by lazy O2 sensors reporting wrong readings — replacing the sensor is a fraction of the cost of a converter. If your scan shows the p0135 code or another oxygen sensor code, check the sensor first with a multimeter or live data, then order the OE part.
C, B & U Codes: Chassis, Body and Network
Beyond the powertrain, the u0100 code, u0101 code, c0035 code and c0040 code cover the network and chassis systems — ABS, wheel speed sensors and module communication. These need a scanner that reads C and B codes, not just P codes.
| Code | Family | Meaning | Likely causes |
|---|---|---|---|
| U0100 code | Network | Lost communication with ECM/PCM | Module power or ground, CAN bus wiring, faulty ECM/PCM |
| U0101 code | Network | Lost communication with TCM | Transmission module power/ground, CAN bus, faulty TCM |
| U0121 code | Network | Lost communication with ABS module | ABS module power/ground, CAN bus wiring, faulty ABS module |
| C0035 code | Chassis | Left front wheel speed sensor circuit | Wheel speed sensor, wiring/connector, tone ring |
| C0040 code | Chassis | Right front wheel speed sensor circuit | Wheel speed sensor, wiring/connector, tone ring |
| U0073 code | Network | Control module communication bus off | CAN bus wiring, terminator, a module pulling the bus down |
| U0140 code | Network | Lost communication with body control module (BCM) | BCM power/ground, CAN bus, faulty BCM |
| U0155 code | Network | Lost communication with instrument panel cluster | Cluster power/ground, CAN bus, faulty cluster |
| U0151 code | Network | Lost communication with restraint control module | RCM power/ground, CAN bus, faulty airbag module |
| C1201 code | Chassis | Engine control system malfunction (ABS-related) | Often a communication or power issue between ABS and engine modules — common on Toyota/Honda |
| B1000 code | Body | ECU malfunction / SRS internal fault | Airbag module internal failure — a B1000 code usually means module replacement, not a sensor |
| B codes | Body | Airbag (SRS), climate and body module faults | SRS faults are the serious ones — an srs code means airbags may not deploy |
A wheel speed sensor code on the ABS system does not stop normal braking — but it does disable ABS and stability control. You need an abs code reader or a scanner that reads C codes to see these; most basic obd2 code readers show P codes only. If the abs light is on, scan the C codes before ordering a sensor — the fault is often the wiring, not the part.
Fault Code Families: How Codes Relate
Many codes come in families — the same fault on Bank 1 vs Bank 2, or the same system with different leak sizes. Reading a code family together points to one root cause instead of several wrong parts.
Fuel trim
P0171 vs P0174 — Too Lean, Both Banks
P0171 is the p0171 bank 1 lean code; P0174 is the same on Bank 2. When both set together, the cause is usually shared — a big vacuum leak, a dirty MAF sensor, or low fuel pressure. One bank only usually means a bank-specific leak or sensor.
Catalyst
P0420 vs P0430 — Catalyst Efficiency
P0420 names Bank 1, P0430 names Bank 2 — the p0430 catalyst system efficiency below threshold bank 2 test is the mirror image. Both codes together point to a shared cause like an exhaust leak or worn O2 sensors ahead of the catalysts.
Misfire
P0300 vs P0301-P0308 — Random vs Single
P0300 is random misfire across cylinders; P0301 through P0308 name one specific cylinder (the p0301 cylinder 1 misfire code, for example). A single-cylinder code swaps coils and plugs; the random code needs a shared-cause check — fuel pressure, vacuum, compression.
EVAP
P0440 / P0442 / P0455 / P0456 — Leak Sizes
The p0442 vs p0455 comparison is leak size: P0442 is a small leak, P0455 a gross leak — and P0456 an even smaller one than P0442. P0440 is the general flag. The smaller the leak, the more likely it is the gas cap; the larger, the more likely a cracked hose or failed purge valve.
How to Read Fault Codes Yourself
How to read fault codes in five steps — from the OBD-II port to a plain-English meaning. This is the same how to read fault codes flow our specialists use, and it takes about five minutes.
Check the Light
Solid = schedule a scan soon. Flashing = stop driving: an active misfire can overheat the catalytic converter. Record whether it flashes under load.
Locate the OBD-II Port
Usually under the dashboard, left of the steering column — a trapezoid connector every car has since 1996.
Plug In a Scanner
Connect the OBD2 scanner with the ignition on, engine off. Wait for the link, then read the codes — see our scanner guide to pick the right one.
Record Freeze Frame Data
The freeze frame data snapshot — RPM, coolant temp, fuel trim and speed at the moment the code set — tells you if the fault happened at idle, load or speed. A good scanner shows it with the code.
Look Up the Code
Match your code to the tables above, or send it to us with your VIN. And remember the pending code meaning: a pending code is a fault that has not run long enough to turn the light on — fix it before it becomes a confirmed code.
Never clear codes before fixing the cause. Clearing turns the light off but the fault remains — the code comes back, often with damage done. After a repair, several warm-up drive cycles are needed before the monitor runs and the light resets by itself.
How to Reset the Check Engine Light — Correctly
How to reset check engine light comes up constantly — but the reset is only correct after the cause is fixed. The check engine light reset process below is the safe way, and the one that actually keeps the light off.
Scanner
Clear Codes with a Scanner
Connect the OBD2 scanner, read the codes, fix the cause, then use Clear Codes — the fastest way to clear check engine light history once the repair is done. The check engine light reset via scanner is immediate — and if the code returns, the fault is not fixed. This is the only method that also clears pending codes.
Battery
Disconnecting the Battery
Disconnecting the battery for 10-15 minutes clears stored codes — but will disconnecting battery reset check engine light permanently? No: it also resets radio presets, clock and adaptive settings, and the light returns if the fault is live. Use it only as a last resort.
Drive cycles
Let the Computer Clear Itself
After a repair, drive the required warm-up drive cycles and the readiness monitors will pass — the light resets by itself. This is how to clear codes without erasing evidence: the freeze frame and history stay for your mechanic.
The reset is not the repair. Clearing codes without fixing the cause turns the light off for a day and hides the history. Diagnose first, fix second, then reset — or send us the code and we will tell you what to fix before you spend anything.
Code not on this list?
Send us the fault code plus your VIN and our specialists will tell you what it means, what part usually fixes it, and which OE part fits your car. We reply within 24 hours, Mon-Sat.
Fault Code Lookup — Frequently Asked Questions
What does the P0420 code mean?
P0420 means catalyst system efficiency below threshold — the catalytic converter is not cleaning exhaust as expected. But do not replace it on the code alone: check O2 sensors and exhaust leaks first, because a lazy sensor or a small leak triggers the same code for a fraction of the repair cost.
Why does P0171 keep coming back?
P0171 (system too lean) returns when the root cause was never fixed — most often a vacuum leak that was masked, a MAF sensor that was cleaned instead of verified, or a weak fuel pump. Check fuel trims on live data: a high positive long-term trim confirms a real lean condition.
Do I need a special scanner for ABS or airbag codes?
Yes. Basic code readers show powertrain P codes only. C codes (ABS) and B codes (airbags/SRS) need a scanner that reads all families — many mid-range OBD2 scanners do, but confirm before buying. See our scanner guide.
How do I know if it is P0171 or P0174?
P0171 is system too lean on Bank 1, P0174 on Bank 2. If both set together, check the shared causes first: a vacuum leak, dirty MAF, or low fuel pressure. One bank only points to a bank-specific leak or sensor.
Can I drive with a P0300 misfire code?
Not for long. A misfire sends raw fuel into the catalytic converter, which can overheat and destroy it. If the check engine light is flashing, stop driving and address the misfire — usually spark plugs, coils or injectors — before continuing.
What is a pending code?
A pending code is a fault the computer has detected but not yet confirmed — it has not failed enough drive cycles to turn the check engine light on. Fix it before it becomes a confirmed code; many pending codes are intermittent wiring faults.
What is the difference between P0420 and P0430?
P0420 is catalyst efficiency below threshold on Bank 1; P0430 is the same test on Bank 2. Both together usually mean a shared cause — an exhaust leak or worn O2 sensors — rather than two failed converters.
How do I get the right O2 sensor for my car?
Send us your VIN and the oxygen sensor code — we will match the correct upstream or downstream OE oxygen sensor, cross-referenced from the OE number stamped on your original part. This is our specialty; most codes like P0135 are fixed by the right sensor fitted correctly.