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Engine Misfire
Causes & Repair Cost
A misfire can be as simple as a worn spark plug or as serious as injector, compression or mechanical damage. A fault-code read and targeted tests prevent replacing parts by guesswork.
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SPARK PLUG REPAIR£50-£150
Common lower-cost cause
COMPRESSION FAULT£1,200+
Internal engine diagnosis and repair
SPARK PLUG REPAIR
£50-£150
Common lower-cost cause
COMPRESSION FAULT
£1,200+
Internal engine diagnosis and repair
Get a engine misfire diagnostic or quoteCheck symptoms first
SYMPTOM TO COST - ESCALATIONUnderstanding The Severity
Understanding The Severity
Of Your Engine Misfire
Understanding the severity of this issue is critical to avoiding unnecessary replacement costs.
LIKELY CAUSE
Spark plugs, coils and wiring are common causes on petrol engines.
WHAT TO DO
Read codes and test the affected cylinder.
REPAIR COST
£50-£220 is the typical cost band at this severity. Confirm the fault before authorising work.
LIKELY CAUSE
Spark plugs, coils and wiring are common causes on petrol engines.
WHAT TO DO
Read codes and test the affected cylinder.
REPAIR COST
£50-£220 is the typical cost band at this severity. Confirm the fault before authorising work.
WHY ENGINE MISFIRE HAPPENS
THE THREE COMMON
FAILURE MODES
Knowing why this fault develops helps distinguish a focused repair from a problem that has already affected the wider engine system.
01
Ignition fault
Spark plugs, coils and wiring are common causes on petrol engines.
02
Fuel injector fault
A blocked, worn or electrically failed injector can misfire a cylinder.
03
Low compression
Valve, gasket, ring or bore faults create a mechanical misfire.
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NoobligationGet a engine misfire diagnostic or quoteCheck symptoms first
CLEAN / REPAIR VS REPLACE -
DECISION FRAMEWORKNOT EVERY ENGINE MISFIRE
DECISION FRAMEWORK
NOT EVERY ENGINE MISFIRE
NEEDS A NEW PART
A proper diagnostic is essential to determine the most economical route.
START WITH THE BASICS IF:
- The misfire has a clear fault code
- Plugs, coils and wiring are due for inspection
- Compression and fuel tests are normal
Many ignition repairs take 1-2 hours
ESCALATE IF:
- The engine light is flashing
- Misfire persists after ignition and injector checks
- Compression testing indicates an internal fault
Internal faults require compression and leak-down diagnosis
CONSEQUENCES OF DELAY
THE DOMINO EFFECT
Ignoring warning signs can turn a contained engine misfire repair into wider engine damage and a much larger bill.
FAULTS CAN SPREAD DOWNSTREAM
Ignition fault
Spark plugs, coils and wiring are common causes on petrol engines.
Fuel injector fault
A blocked, worn or electrically failed injector can misfire a cylinder.
Low compression
Valve, gasket, ring or bore faults create a mechanical misfire.
THE COST ESCALATION
Early repairSpark plugs
£50-£150
If delayedIgnition coils
£70-£220
Major damageInjector / compression fault
£120-£1,200+Addressing the underlying fault early can prevent more expensive secondary repairs.
RED FLAG ENGINES -
KNOWN FAILURE PATTERNS
Vehicles that warrant a prompt specialist diagnosis.
| ENGINE / VEHICLE | YEARS | RISK |
|---|---|---|
| Direct-injection petrol | 2008 onwards | Coil, plug and injector faults |
| Common-rail diesel | 2005 onwards | Injector and fuel pressure faults |
| High-mileage engines | All years | Compression-related misfire |
VEHICLE VALUE
THRESHOLD RULEIS REPAIR
THRESHOLD RULE
IS REPAIR
ECONOMICAL?
Compare the viable repair cost with the vehicle's current market value before committing.
| VEHICLE VALUE | RECOMMENDED ACTION | REASONING |
|---|---|---|
| Lower-value vehicle | Start with diagnosis and the lowest viable repair | A major repair can exceed a sensible share of the vehicle value. |
| Higher-value vehicle | Repair or replacement may be viable | A verified repair can restore reliability and value. |
Do not commit to major work until the fault and total repair route have been confirmed.
COMPLETE ORIGINAL GUIDE DATA
In-Depth Engine Misfire Information
Page-specific diagnosis, cost, decision and safety information extracted from the original static guide.
How fast damage escalates+
Click any stage to see full detail. A flashing EML is an emergency.
Worn spark plugs or minor ignition fault. The most common and cheapest fix on petrol engines.
Book a diagnostic. If spark plugs are due (60k-80k miles), replace them. Cost: £50-£150 for full set.
£50-£150 for spark plugs. Labour: 1-2 hours at £65-£95/hr.
Failed ignition coil (petrol) or fuel injector (petrol/diesel). Constant misfire on a specific cylinder.
Swap ignition coils to see if misfire moves. If yes, replace coil (£70-£220). If no, test injectors.
£70-£220 for coil. £120-£900 for injector. Labour: 1-4 hours.
Multiple cylinders misfiring - unburnt fuel destroying catalytic converter or DPF.
Stop driving immediately. Call recovery. Do not continue - cat/DPF damage is occurring.
£150-£600 for ignition components + possible £500-£2,000 for cat/DPF replacement.
Low compression - valves, piston rings, or head gasket failure. Internal engine damage.
Full diagnostic - compression test and leak-down test. Repair or replace engine based on vehicle value.
£600-£3,500+ for engine repair or head rebuild. Labour: 8-20 hours.
Severe internal damage from ignored misfire - bearings destroyed, pistons damaged.
Do not attempt to restart. Call recovery. Engine replacement is the only option.
£2,500-£6,500+ for reconditioned engine replacement. On lower-value vehicles, this often means scrapping the car.
Click any stage to see full detail. A flashing EML is an emergency. £50 Slight rough idle, occasional hesitation Worn spark plugs or minor ignition fault £50 - £150 (spark plugs) £70 Constant misfire on one cylinder, EML steady Failed ignition coil or injector £70 - £220 (coil) or £120 - £900 (injector) £150 Flashing EML, strong misfire, vibration, poor fuel economy Multiple cylinders misfiring - cat/DPF being destroyed £150 - £600 + possible cat/DPF damage £600 Engine barely runs, no power, failed emissions Low compression - valves, rings, or head gasket £600 - £3,500+ £6,500+ Engine knock, metal in oil, seizure Severe internal damage from ignored misfire £2,500 - £6,500+ Likely cause Worn spark plugs or minor ignition fault. The most common and cheapest fix on petrol engines. What to do Book a diagnostic. If spark plugs are due (60k-80k miles), replace them. Cost: £50-£150 for full set. Repair cost £50-£150 for spark plugs. Labour: 1-2 hours at £65-£95/hr. Likely cause Failed ignition coil (petrol) or fuel injector (petrol/diesel). Constant misfire on a specific cylinder. What to do Swap ignition coils to see if misfire moves. If yes, replace coil (£70-£220). If no, test injectors. Repair cost £70-£220 for coil. £120-£900 for injector. Labour: 1-4 hours. Likely cause Multiple cylinders misfiring - unburnt fuel destroying catalytic converter or DPF. What to do Stop driving immediately. Call recovery. Do not continue - cat/DPF damage is occurring. Repair cost £150-£600 for ignition components + possible £500-£2,000 for cat/DPF replacement. Likely cause Low compression - valves, piston rings, or head gasket failure. Internal engine damage. What to do Full diagnostic - compression test and leak-down test. Repair or replace engine based on vehicle value. Repair cost £600-£3,500+ for engine repair or head rebuild. Labour: 8-20 hours. Likely cause Severe internal damage from ignored misfire - bearings destroyed, pistons damaged. What to do Do not attempt to restart. Call recovery. Engine replacement is the only option. Repair cost £2,500-£6,500+ for reconditioned engine replacement. On lower-value vehicles, this often means scrapping the car. Signs of engine misfire
Don't ignore these warning signs+
🔶 EARLY WARNING (Book diagnostic soon) • Rough idle - engine shakes or vibrates when stationary • Slight hesitation when accelerating • Loss of power, especially uphill or under load • Poor fuel economy - noticeable drop in MPG • Engine management light steady (not flashing) 🔴 PROGRESSED DAMAGE (Stop driving - immediate action) • Flashing engine management light - cat/DPF damage occurring • Constant misfire on one or more cylinders • Engine vibration at all RPMs • Failed MOT emissions - high hydrocarbons (HC) • Petrol smell from exhaust - unburnt fuel • Engine barely runs or won't start Common causes of engine misfire
- • Rough idle - engine shakes or vibrates when stationary
- • Slight hesitation when accelerating
- • Loss of power, especially uphill or under load
- • Poor fuel economy - noticeable drop in MPG
- • Engine management light steady (not flashing)
- • Flashing engine management light - cat/DPF damage occurring
- • Constant misfire on one or more cylinders
- • Engine vibration at all RPMs
- • Failed MOT emissions - high hydrocarbons (HC)
- • Petrol smell from exhaust - unburnt fuel
- • Engine barely runs or won't start
Cheapest to most expensive+
Market context: Based on Engines Market enquiry volumes, BMW N47D20C (1,450 requests [EM-VERIFIED]), Mercedes OM651 (1,250), VW/Audi EA189 (1,050), and Ford DV6 (780) are among the most frequently requested engines. Misfire complaints on these engines are often traced to injector issues (diesel) or ignition components (petrol). [EM-VERIFIED]
| Cause | How It Happens | Typical Repair Cost | Most Common On |
|---|---|---|---|
| Spark plugs (worn) | Electrode wears out - weak or no spark | £50 - £150 (full set) | All petrol engines, especially high-mileage |
| Ignition coil failure | Coil internal failure - no spark to cylinder | £70 - £220 (one coil) | Ford EcoBoost, VW TSI, BMW N-series |
| Fuel injector (clogged/failed) | Injector stuck open/closed - incorrect fuel delivery | £120 - £900 (per injector) | Diesel engines, direct-injection petrols |
| Vacuum / air leak | Cracked hose or intake gasket - unmetered air | £50 - £300 | All engines, especially older models |
| EGR valve stuck open | Exhaust gas recirculates at wrong time | £135 - £650 | Diesel engines, VW EA189, Mercedes OM651 |
| Low compression (valves) | Burnt or bent valve - cylinder can't seal | £800 - £2,000 | High-mileage petrol, interference engines |
| Low compression (rings) | Worn rings - oil in cylinder, poor sealing | £1,500 - £3,500 | High-mileage engines, neglected maintenance |
| Head gasket failure | Gasket blown between cylinders or into coolant | £550 - £1,200 | BMW N47, Subaru, VW EA189, Mercedes OM651 |
Finding the cause - cheapest to most expensive+
Step Test What It Tells You Typical Cost 1 OBD-II scan Fault codes identify which cylinder £30 - £80 2 Swap ignition coils (petrol) If misfire moves with coil → bad coil £0 - £50 diagnostic 3 Swap spark plugs (petrol) If misfire moves with plug → bad plug £50 - £150 (new plugs) 4 Injector test Flow test - confirms injector failure £120 - £900 (replacement) 5 Vacuum / smoke test Identifies air leaks £50 - £150 6 Compression test Measures cylinder pressure - low = internal damage £80 - £150 7 Leak-down test Identifies where compression escapes £150 - £250 8 Borescope inspection Visual of cylinder walls, piston, valves £50 - £150 Specialist rule of thumb: "On a petrol engine with a single-cylinder misfire, swap the ignition coil with another cylinder. If the misfire moves, it's a £70-£220 coil. If it doesn't move, it could be a spark plug, injector, or compression. Diagnose from cheapest to most expensive - always." Diagnostic decisions
| Step | Test | What It Tells You | Typical Cost |
|---|---|---|---|
| 1 | OBD-II scan | Fault codes identify which cylinder | £30 - £80 |
| 2 | Swap ignition coils (petrol) | If misfire moves with coil → bad coil | £0 - £50 diagnostic |
| 3 | Swap spark plugs (petrol) | If misfire moves with plug → bad plug | £50 - £150 (new plugs) |
| 4 | Injector test | Flow test - confirms injector failure | £120 - £900 (replacement) |
| 5 | Vacuum / smoke test | Identifies air leaks | £50 - £150 |
| 6 | Compression test | Measures cylinder pressure - low = internal damage | £80 - £150 |
| 7 | Leak-down test | Identifies where compression escapes | £150 - £250 |
| 8 | Borescope inspection | Visual of cylinder walls, piston, valves | £50 - £150 |
Interpreting fault codes+
Fault Code Meaning Most Likely Cause Typical Cost P0300 Random/multiple cylinder misfire Vacuum leak, fuel delivery, or compression £50 - £3,500+ P0301 Cylinder 1 misfire Coil, plug, injector, or compression £50 - £3,500+ P0302-P0308 Cylinder-specific misfire Coil, plug, injector, or compression £50 - £3,500+ P0171 / P0174 Lean mixture Vacuum leak, MAF sensor, fuel delivery £50 - £500 P0201-P0208 Injector circuit fault Failed injector or wiring £120 - £900 Urgency matrix: EML off, occasional hesitation Book diagnostic within 2 weeks EML steady on, car runs rough Book diagnostic within days EML flashing Stop driving immediately Red flag engines - known misfire patterns
| Fault Code | Meaning | Most Likely Cause | Typical Cost |
|---|---|---|---|
| P0300 | Random/multiple cylinder misfire | Vacuum leak, fuel delivery, or compression | £50 - £3,500+ |
| P0301 | Cylinder 1 misfire | Coil, plug, injector, or compression | £50 - £3,500+ |
| P0302-P0308 | Cylinder-specific misfire | Coil, plug, injector, or compression | £50 - £3,500+ |
| P0171 / P0174 | Lean mixture | Vacuum leak, MAF sensor, fuel delivery | £50 - £500 |
| P0201-P0208 | Injector circuit fault | Failed injector or wiring | £120 - £900 |
Engines with known misfire issues+
Based on KNOWN FAILURE PATTERNS and EM_Proprietary_Data_2025.
Based on KNOWN FAILURE PATTERNS and EM_Proprietary_Data_2025. Mercedes OM651 60k-120k 1,250 [EM-VERIFIED] Injector seal failure (Black Death) - carbon buildup. Seals: £400-£1,200; Injector: £300-£600. BMW N47D20C 60k-120k 1,450 [EM-VERIFIED] Injector failure (secondary to chain debris). Injector: £300-£800. VW/Audi EA189 2.0 TDI 80k-150k 1,050 [EM-VERIFIED] Injector coking - Dieselgate software increased failure rate. Cleaning: £200-£500; Injector: £300-£800. Ford DV6 1.6 TDCi 80k-150k 780 [EM-VERIFIED] Injector failure - very common on high-mileage. Injector: £300-£800. VW/Audi EA888 Gen3 2.0 TSI 60k-100k 130 [EM-VERIFIED] Ignition coil failure - common on petrol. Coil: £70-£150; Spark plugs: £50-£100. Ford EcoBoost 1.0 (early) 60k-100k 185 [EM-VERIFIED] Wet belt debris - misfire from low oil pressure affecting valve train. Belt replacement: £600-£1,500. Market Intelligence (Mercedes OM651): The OM651 engine generated 1,250 quote requests in 2025 [EM-VERIFIED n=1,250]. Specialists report that injector seal failure ("Black Death") - carbon build-up around injectors caused by leaking seals - is the most common failure on this engine. If caught early, seal replacement costs £400-£1,200. Ignored, the injector seizes in the head, requiring specialist removal or cylinder head replacement. [KNOWN FAILURE PATTERNS] Flashing EML - why you must stop immediately
Driving with a flashing EML+
Time Driven with Flashing EML Damage Cost to Fix 1-2 minutes Unburnt fuel reaches cat/DPF Minor - may clear with driving 5-10 minutes Cat overheats - substrate begins melting £500 - £1,200 20+ minutes Cat/DPF destroyed, possible engine damage £1,000 - £2,500 + engine repair Extended driving Bore wash - fuel washes oil from cylinder walls £2,500 - £6,500+ Real-world example: A 2014 Ford Focus 1.6 EcoBoost owner saw a flashing engine management light but drove 15 miles home. The constant misfire dumped unburnt fuel into the catalytic converter, melting the internal substrate. Total cost: £120 (ignition coil) + £1,100 (catalytic converter) = £1,220. Stopping immediately would have been a £120 coil replacement. Repair options - decision framework
| Time Driven with Flashing EML | Damage | Cost to Fix |
|---|---|---|
| 1-2 minutes | Unburnt fuel reaches cat/DPF | Minor - may clear with driving |
| 5-10 minutes | Cat overheats - substrate begins melting | £500 - £1,200 |
| 20+ minutes | Cat/DPF destroyed, possible engine damage | £1,000 - £2,500 + engine repair |
| Extended driving | Bore wash - fuel washes oil from cylinder walls | £2,500 - £6,500+ |
What should you do-+
Cause Repair Option Typical Cost When to Choose Worn spark plugs Spark plug replacement (full set) £50 - £150 Every 60k-80k miles on petrol Failed ignition coil Coil replacement (one cylinder) £70 - £220 If misfire moves with coil swap Failed injector Injector replacement (one) - reconditioned or new £120 - £900 If injector confirmed by diagnostic Vacuum / air leak Hose or gasket replacement £50 - £300 If lean codes present EGR valve stuck open EGR cleaning or replacement £135 - £650 If misfire only at idle Low compression (valves) Valve job (head removal, valve replacement) £800 - £2,000 Viable on vehicles worth £2,500+ Low compression (rings) Engine rebuild or replacement £1,500 - £6,500+ Only on vehicles worth £3,000+ Head gasket failure Head gasket replacement £550 - £1,200 Viable on most vehicles worth £2,000+ Specialist rule of thumb (petrol): "On a petrol engine with a misfire, 70% of the time it's a coil or spark plug. Swap the coil first - it's free to test. If the misfire moves, buy one coil for £70-£150. Don't buy a full set unless they're all old." Specialist rule of thumb (diesel): "On a diesel with a misfire, 80% of the time it's an injector. But before you replace injectors, do a compression test. A dead cylinder from low compression will act like a bad injector, and a new injector won't fix it." Preventative maintenance
| Cause | Repair Option | Typical Cost | When to Choose |
|---|---|---|---|
| Worn spark plugs | Spark plug replacement (full set) | £50 - £150 | Every 60k-80k miles on petrol |
| Failed ignition coil | Coil replacement (one cylinder) | £70 - £220 | If misfire moves with coil swap |
| Failed injector | Injector replacement (one) - reconditioned or new | £120 - £900 | If injector confirmed by diagnostic |
| Vacuum / air leak | Hose or gasket replacement | £50 - £300 | If lean codes present |
| EGR valve stuck open | EGR cleaning or replacement | £135 - £650 | If misfire only at idle |
| Low compression (valves) | Valve job (head removal, valve replacement) | £800 - £2,000 | Viable on vehicles worth £2,500+ |
| Low compression (rings) | Engine rebuild or replacement | £1,500 - £6,500+ | Only on vehicles worth £3,000+ |
| Head gasket failure | Head gasket replacement | £550 - £1,200 | Viable on most vehicles worth £2,000+ |
Avoiding misfires+
Prevention Method Cost Effectiveness Frequency Spark plug replacement (petrol) £50 - £150 Very high Every 60k-80k miles Ignition coil inspection £0 - £50 Medium Every service on high-mileage petrol Fuel system cleaner (diesel) £10 - £20 Medium Every 10k-15k miles Use quality fuel +5-10p per litre Medium Every 3-4 fill-ups Regular oil changes (correct spec) £50 - £150 High Every 6,000-10,000 miles Address EML immediately £0 - £50 diagnostic Very high When light appears Specialist rule of thumb: "The most common cause of a flashing EML on a modern petrol car is a failed ignition coil. The most common cause on a diesel is a failed injector. In both cases, ignoring it will cost you a catalytic converter or DPF on top of the original repair." Frequently asked questions
| Prevention Method | Cost | Effectiveness | Frequency |
|---|---|---|---|
| Spark plug replacement (petrol) | £50 - £150 | Very high | Every 60k-80k miles |
| Ignition coil inspection | £0 - £50 | Medium | Every service on high-mileage petrol |
| Fuel system cleaner (diesel) | £10 - £20 | Medium | Every 10k-15k miles |
| Use quality fuel | +5-10p per litre | Medium | Every 3-4 fill-ups |
| Regular oil changes (correct spec) | £50 - £150 | High | Every 6,000-10,000 miles |
| Address EML immediately | £0 - £50 diagnostic | Very high | When light appears |
FREQUENTLY ASKED QUESTIONS
Common Questions Answered
1How much does engine misfire repair cost in the UK+
Spark plugs (full set): £50-£150. Ignition coil (one): £70-£220. Fuel injector (one): £120-£900. Vacuum leak repair: £50-£300. EGR valve: £135-£650. Low compression (valves): £800-£2,000. Low compression (rings): £1,500-£3,500. Head gasket: £550-£1,200. Engine replacement: £2,500-£6,500+. Diagnostic: £50-£150. → Engine repair cost guide
2What causes an engine to misfire+
Most common: worn spark plugs (petrol), failed ignition coil (petrol), clogged or failed fuel injector (petrol and diesel), vacuum/air leak, EGR valve stuck open (diesel), low compression (valves, rings, head gasket).
3Can I drive with a misfire+
If the engine management light is steady and the car runs reasonably, you can drive gently to a garage. If the light is flashing, stop driving immediately - unburnt fuel is destroying your catalytic converter (petrol) or DPF (diesel). A flashing EML is an emergency.
4How do I know if it's a coil or spark plug+
Swap the ignition coil from the misfiring cylinder with another cylinder. If the misfire moves to the new cylinder, the coil is bad (£70-£220). If the misfire stays on the same cylinder, it could be a spark plug, injector, or compression issue. Diagnose from cheapest to most expensive.
5Is it worth fixing a misfire on an old car+
If your car is worth £2,000-£4,000 and the repair is a coil (£70-£220) or injector (£120-£900), yes - a replacement car would cost far more. If the misfire is caused by low compression (valves: £800-£2,000) and the car is worth under £2,000, evaluate scrap first. → Repair vs replacement guide
6Which engines have the most misfire problems+
Based on 2025 data and KNOWN FAILURE PATTERNS: Mercedes OM651 (1,250 - injector seal failure), VW/Audi EA189 (1,050 - injector coking), BMW N47 (1,450 - injector issues from chain debris), Ford DV6 (780 - injector failure). On petrol engines, VW/Audi EA888 (130 - ignition coil failure) and Ford EcoBoost (175 - coil or injector issues). [EM-VERIFIED]
Engine Misfire repair cost
DPF failure and cleaning guide
Engine diagnostics service
Repair vs replacement decision toolROUGH RUNNING, VIBRATION OR AN ENGINE LIGHT?
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