Complete OBD2 Fault Code Guide: What Every Code Means & When to Worry

That amber check engine light has just come on. Your stomach drops. Is it serious? Is it safe to keep driving? Will it cost a fortune to fix? These are the questions every driver asks, and the answer lies in the OBD2 fault code stored in your car's computer.

OBD2 (On-Board Diagnostics, version 2) has been mandatory on all petrol cars sold in the UK since 2001 and all diesel cars since 2004. The system monitors dozens of sensors across your engine, transmission, emissions, and ancillary systems. When something falls outside expected parameters, it logs a standardised fault code and often illuminates the check engine light (MIL — Malfunction Indicator Lamp).

This guide explains the code structure, lists the 20 most common codes you are likely to encounter, and gives you the information you need to decide whether to keep driving, book a garage appointment, or pull over immediately.

Understanding the Code Structure

Every OBD2 fault code follows the same five-character format. Understanding this structure helps you quickly assess what system is affected.

First Character: The System

• P — Powertrain (engine, transmission, fuel system, ignition). By far the most common type.
• B — Body (airbags, seatbelts, climate control, interior electronics).
• C — Chassis (ABS, traction control, steering, suspension electronics).
• U — Network/Communication (CAN bus faults, module communication errors).

Second Character: Generic vs Manufacturer-Specific

• 0 — Generic (SAE standard). These codes mean the same thing on every car, regardless of make.
• 1 — Manufacturer-specific. The meaning may vary between manufacturers. For example, P1xxx on a Ford means something different from P1xxx on a BMW.

Remaining Characters: Subsystem and Fault

The third character identifies the specific subsystem (1 = fuel/air metering, 2 = fuel/air metering (injector circuit), 3 = ignition system, 4 = auxiliary emissions, 5 = vehicle speed/idle control, 6 = computer output circuits, 7/8 = transmission). The final two digits identify the specific fault.

For example: P0171 breaks down as P (Powertrain) + 0 (Generic) + 1 (Fuel/Air Metering) + 71 (System Too Lean, Bank 1).

Top 20 Most Common Fault Codes

These are the codes you are most likely to encounter on UK roads. For each, we provide the meaning, severity, typical repair cost, and whether it is safe to continue driving.

P0300 — Random/Multiple Cylinder Misfire Detected

Severity: High | Repair cost: £100–£600 | Safe to drive: No (if flashing MIL)

The engine is misfiring across multiple cylinders. Causes include worn spark plugs, faulty ignition coils, vacuum leaks, low fuel pressure, or a failing head gasket. A flashing check engine light during acceleration means catalytic converter damage is imminent — stop driving. A steady light is less urgent but still requires prompt attention.

P0171 — System Too Lean (Bank 1)

Severity: Medium | Repair cost: £80–£400 | Safe to drive: Yes, with caution

The engine is running with too much air relative to fuel. Common causes: vacuum leak (split intake hose, leaking gasket), weak fuel pump, clogged fuel filter, dirty or faulty MAF (Mass Air Flow) sensor. Often accompanied by P0174 (Bank 2 lean). Cleaning the MAF sensor (£10 for cleaner spray) fixes this roughly 30% of the time.

P0420 — Catalyst System Efficiency Below Threshold (Bank 1)

Severity: Medium | Repair cost: £300–£1,200 | Safe to drive: Yes

The catalytic converter is not performing efficiently enough. This could mean the cat is genuinely failing (common after 80,000–120,000 miles) or it could be a faulty downstream oxygen sensor giving incorrect readings. Always check the O2 sensor first (£80–£150) before committing to a cat replacement. Will cause an MOT emissions failure.

P0440 — Evaporative Emission Control System Malfunction

Severity: Low | Repair cost: £50–£250 | Safe to drive: Yes

The EVAP system, which prevents fuel vapours from escaping into the atmosphere, has a fault. Often caused by a loose or damaged fuel cap (try tightening it first — free fix). Can also indicate a cracked charcoal canister or faulty purge valve.

P0128 — Coolant Thermostat Below Regulating Temperature

Severity: Low–Medium | Repair cost: £80–£200 | Safe to drive: Yes

The engine is not reaching operating temperature quickly enough. Almost always a stuck-open thermostat. The car will use more fuel and the heater may blow lukewarm air. Not urgent but should be fixed before winter and will affect emissions. Thermostat replacement is straightforward on most cars.

P0455 — Evaporative Emission System Leak Detected (Gross Leak)

Severity: Low | Repair cost: £0–£200 | Safe to drive: Yes

A large leak in the EVAP system. Check your fuel cap first — it may be loose, cracked, or missing its seal. If the cap is fine, the leak is likely in a hose, the charcoal canister, or the purge/vent valve. No driveability issues but will cause an MOT emissions failure.

P0301–P0304 — Cylinder 1–4 Misfire Detected

Severity: High | Repair cost: £60–£400 | Safe to drive: With caution (not if flashing MIL)

A misfire in a specific cylinder. The last digit indicates which cylinder: P0301 = cylinder 1, P0302 = cylinder 2, and so on. Most commonly caused by a failed ignition coil (£40–£100 each) or worn spark plug (£8–£25 each). Swap the coil from the misfiring cylinder with another — if the misfire follows the coil, you have found the problem.

P0141 — O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 2)

Severity: Low–Medium | Repair cost: £80–£180 | Safe to drive: Yes

The downstream oxygen sensor's heater element has failed. The sensor itself may still work once warmed up by exhaust heat, but cold-start emissions will be elevated. Straightforward replacement on most cars.

P0442 — Evaporative Emission System Leak Detected (Small Leak)

Severity: Low | Repair cost: £50–£200 | Safe to drive: Yes

Similar to P0455 but indicating a smaller leak. Harder to find. Professional garages use a smoke machine to locate the leak. Often a deteriorated hose or seal in the EVAP system.

P0401 — Exhaust Gas Recirculation Flow Insufficient

Severity: Medium | Repair cost: £100–£350 | Safe to drive: Yes

The EGR (Exhaust Gas Recirculation) valve is not flowing enough exhaust gas back into the intake. Usually caused by carbon buildup blocking the EGR valve or its passages. Cleaning is often possible (£100–£150), otherwise replacement (£200–£350). Common on diesel vehicles. Will cause MOT emissions failure.

P0505 — Idle Air Control System Malfunction

Severity: Medium | Repair cost: £80–£250 | Safe to drive: Yes, but may stall

The idle control system cannot maintain correct idle speed. Symptoms include erratic idle, stalling at junctions, or unusually high idle speed. Often caused by a dirty or failed idle air control valve (IACV). Cleaning with carburettor cleaner works in many cases.

P0340 — Camshaft Position Sensor Circuit Malfunction

Severity: High | Repair cost: £80–£250 | Safe to drive: With caution

The ECU is not receiving a signal from the camshaft position sensor. This can cause rough running, hard starting, or stalling. The sensor itself is usually inexpensive (£20–£60) but diagnosis should confirm whether the issue is the sensor, wiring, or timing chain stretch affecting the signal pattern.

P0500 — Vehicle Speed Sensor Malfunction

Severity: Medium–High | Repair cost: £60–£200 | Safe to drive: With caution

The ECU is not receiving a vehicle speed signal. This can affect the speedometer, ABS, traction control, and automatic transmission shift points. Often a failed sensor or damaged wiring near the gearbox.

P0700 — Transmission Control System Malfunction

Severity: High | Repair cost: £100–£2,000+ | Safe to drive: With caution

This is a general code indicating that the transmission control module (TCM) has logged its own fault. It is always accompanied by more specific transmission codes that pinpoint the actual problem. Read the additional codes before any diagnosis.

P0113 — Intake Air Temperature Sensor High Input

Severity: Low | Repair cost: £40–£120 | Safe to drive: Yes

The intake air temperature (IAT) sensor is reporting an implausibly high reading. Usually a failed sensor or broken wire. The ECU will use a default value, so driveability is rarely affected. Cheap and easy fix.

P0135 — O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)

Severity: Medium | Repair cost: £100–£200 | Safe to drive: Yes

The upstream oxygen sensor's heater has failed. This sensor is more critical than the downstream one (P0141) because it directly affects fuelling. Cold-start running will be rough and fuel consumption will increase until the sensor warms up naturally.

P0325 — Knock Sensor 1 Circuit Malfunction

Severity: Medium | Repair cost: £100–£300 | Safe to drive: Yes, but avoid heavy acceleration

The knock sensor detects engine detonation (pinking) and retards ignition timing to protect the engine. Without it, the ECU will run conservative timing, reducing power and fuel efficiency. Replacement is straightforward but the sensor is sometimes awkwardly located beneath the intake manifold.

P0446 — Evaporative Emission System Vent Control Circuit

Severity: Low | Repair cost: £60–£180 | Safe to drive: Yes

The EVAP vent valve or its circuit has a fault. This valve allows air into the charcoal canister during purging. No driveability symptoms but will illuminate the check engine light and affect emissions.

P0102 — Mass Air Flow Sensor Circuit Low Input

Severity: Medium | Repair cost: £40–£250 | Safe to drive: Yes, with reduced performance

The MAF sensor is reporting lower-than-expected airflow. Often caused by a dirty sensor element (try cleaning with MAF cleaner spray, £8–£12) or a damaged sensor wire. Can also indicate an air leak after the MAF sensor. A faulty MAF causes poor fuel economy, hesitation, and rough idle.

P0341 — Camshaft Position Sensor Range/Performance

Severity: High | Repair cost: £80–£1,500 | Safe to drive: With caution

The camshaft position sensor is giving readings outside the expected range. This code is more concerning than P0340 because it can indicate timing chain stretch or a jumped timing belt, rather than a simple sensor failure. Diagnosis should include checking timing chain/belt condition before replacing the sensor. If the timing has jumped, continued driving risks catastrophic engine damage.

When to Worry vs When to Monitor

Pull Over Immediately

• Flashing check engine light (active misfire causing catalyst damage)
• Red oil pressure warning light
• Red coolant temperature warning light
• Loss of power combined with unusual noises or smoke

Book a Garage Within a Week

• Steady check engine light with noticeable symptoms (rough idle, hesitation, stalling)
• Misfire codes (P0300–P0312)
• Transmission codes (P0700+)
• Speed sensor codes affecting ABS/traction control

Monitor and Schedule at Convenience

• EVAP codes (P0440, P0442, P0446, P0455) — no driveability impact
• Single oxygen sensor heater codes
• Intake air temperature sensor codes
• Thermostat codes (P0128) — unless winter is approaching

Best OBD2 Scanners for UK Cars

Having your own OBD2 scanner saves money and gives you immediate information when a warning light appears. Here are our recommendations across different budgets:

Scanner	Price	Best For	Key Features
ELM327 Bluetooth Clone	£8–£15	Budget basics	Read/clear codes, basic live data. Pair with free Torque app (Android) or OBD Fusion (iOS)
Vgate iCar Pro	£18–£28	Reliable budget option	BLE 4.0 for iOS compatibility, faster than cheap ELM327 clones
OBDLink LX	£25–£35	Enthusiast standard	Genuine ELM327 chip, fast data rates, works with all major apps
ANCEL AD310	£20–£30	Standalone handheld	No phone needed, built-in screen, read/clear codes, basic data
Foxwell NT301	£40–£55	Best value handheld	Colour screen, live data, I/M readiness, freeze frame data
BlueDriver	£70–£90	Best app-based scanner	Enhanced codes for many manufacturers, repair reports, smog check
Launch CRP123X	£120–£160	Multi-system diagnosis	Engine, ABS, SRS, transmission. Good coverage of European cars
iCarsoft CR V3.0	£180–£250	European car specialist	Full system scan, service resets, coding capabilities for BMW, VW, Mercedes
Autel AP200	£50–£70	Bluetooth + full systems	All-system scan via phone app, includes 1 free vehicle software
Thinkdiag	£40–£60	Budget full-system	Bluetooth, full system scan, active tests. Annual subscription for updates

How AI Mechanic Can Help

Reading a fault code is only half the battle — understanding what it means for your specific car is what matters. AI Mechanic combines fault code data with your vehicle's make, model, age, and mileage to provide contextual diagnosis.

Simply enter your registration number and fault code at aimechanic.uk. Our AI diagnostic engine will tell you:

• What the code means in plain English
• How serious it is for your specific vehicle
• The most likely causes, ranked by probability for your make and model
• Estimated repair costs at UK garages
• Whether it is safe to continue driving
• What questions to ask your mechanic

No jargon, no guesswork. Just clear, actionable advice from an engine that understands the patterns across millions of diagnostic records.