Quick Answer: Bad crankshaft position sensor (CPS) = one of 7 signs: (1) check engine light (codes P0335, P0336), (2) engine won’t start (no signal = no fuel/spark), (3) rough idling (misfires from timing issues), (4) stalling while driving (intermittent signal dropout), (5) poor acceleration (ECU can’t calculate timing), (6) ticking/knocking (timing way off), (7) limp mode engagement (ECU defaults to safe mode). Cost if caught immediately: £120–£250 (sensor replacement). Cost if ignored 1 week: £250–£450 (fuel system stress, possible transmission strain). Cost if ignored 1 month: £600–£1,500+ (catalytic converter damage from misfires, engine damage from advanced timing). Timeline: CPS failure immediate impact. No start condition appears first. Then stalling/rough running. Finally engine damage if ignored. Safety risk: Engine cuts out while driving = dangerous (especially motorway).
Why This Matters — Crankshaft Sensor Failure Disables Your Engine
Most drivers think: “Engine won’t start, probably battery or starter issue.”
Actually different. CPS failure = engine disabled.
Bad crankshaft sensor → engine computer can’t read crank position → can’t calculate fuel injection timing → can’t determine spark timing → no fuel spray → no spark → engine won’t start or stalls immediately.
Real scenario: Your Toyota Corolla, 2011, 156,000 km. One morning, engine cranks but won’t start (keeps trying to turn over, doesn’t catch). You assume dead battery, call breakdown. Mechanic tests: battery fine, starter working. Connects scanner: code P0335 (crankshaft sensor). Sensor dead (no signal). Replacement: sensor £95 + labor £85 = £180. Breakdown tow: £150. Total: £330. If you’d replaced sensor preventively when seeing earlier warning signs, you’d have avoided tow charge + no-start emergency.
Ignoring sensor failure = £330 emergency instead of £95 preventive fix.
Crankshaft Position Sensor — How It Works
What it is:
- Hall effect or magnetic pickup sensor
- Mounted near crankshaft (usually on engine block or timing cover)
- Detects crankshaft rotation position
- Sends signal to Engine Control Unit (ECU) thousands of times per minute
What it does:
- Reads crankshaft position continuously (TDC = Top Dead Center reference)
- Tells ECU exactly when piston reaches top
- ECU uses this to calculate: fuel injection timing, spark plug timing, transmission shift points
- All engine timing depends on this one sensor
Why it’s critical:
- If ECU doesn’t know crank position: can’t inject fuel (no start)
- If signal is late/early: timing wrong (misfires, rough running)
- If signal drops: ECU goes into safe mode (limp mode = power loss)
The 7 Real Symptoms — Detailed
Symptom #1: Check Engine Light + P0335/P0336 Codes (Most Reliable — 50%)
What you see:
- Dashboard check engine light ON
- Diagnostic codes when scanned:
- P0335 = Crankshaft position sensor circuit
- P0336 = Crankshaft position sensor range/performance
- P0337 = Crankshaft sensor low voltage
What’s happening:
- ECU detected sensor signal missing or out of range
- Most reliable indicator (computer detected actual problem)
- No guessing required — code points directly to CPS
Real example: Ford Focus, 2013, 124,000 km. Check engine light appears (no drivability issues yet). Owner scans code: P0335 (crankshaft sensor circuit). Mechanic tests: sensor voltage stuck at 0V (should vary with crank rotation). Sensor dead. Replacement: sensor £110 + labor £75 = £185. After: light clears, engine starts normal, runs smooth.
Cost to fix: £120–£250 Urgency: 🟡 MEDIUM (within 1 week)
Symptom #2: Engine Won’t Start (Second — 30%)
What you experience:
- Turn key: engine cranks but won’t catch/fire
- Starter motor works (you hear it turning)
- Engine just keeps cranking (no combustion)
- May try 10+ times before giving up
What’s happening:
- Without crank signal, ECU can’t inject fuel
- Without crank signal, ECU can’t trigger spark
- No fuel + no spark = no combustion = no start
Real example: BMW 316i, 2012, 143,000 km. Car won’t start (engine cranks, won’t turn over). Owner assumes battery. Calls breakdown (£150). Mechanic: battery fine, starter fine. Connects scanner: P0335 code. Crankshaft sensor dead. Replacement: sensor £140 + labor £90 + breakdown charge £150 = £380. Preventive replacement would have cost £140 only = saved £240.
Cost to fix: £120–£250 + potential tow £100–£150 Urgency: 🔴 CRITICAL (IMMEDIATE — car won’t run)
Symptom #3: Rough Idling with Misfires (Third — 15%)
What you experience:
- At traffic light: engine shaking/vibrating
- Misfiring sensation (like cylinders not firing evenly)
- Check engine light may be on (misfire codes P0300-P0308)
- Engine sounds rough/choppy at idle
What’s happening:
- CPS signal degraded (weak or intermittent)
- Timing calculation off (ECU guessing at timing)
- Some cylinders fire at wrong time (misfire)
- Multiple misfires = rough idle
Real example: Vauxhall Astra, 2014, 98,000 km. At traffic light: rough idle, engine shaking. Also check engine light on. Scanner shows: P0303 (cylinder 3 misfire) + P0336 (crankshaft sensor performance). CPS signal erratic. Replacement: sensor £120 + labor £80 = £200. After: smooth idle, misfire codes clear.
Cost to fix: £120–£250 Urgency: 🟡 MEDIUM (within 1 week)
Symptom #4: Stalling While Driving (Fourth — 8%)
What you experience:
- Engine suddenly shuts off while driving
- Usually at low speed (traffic light, parking lot)
- Sometimes restarts immediately, sometimes doesn’t
- Very dangerous if happens on motorway
What’s happening:
- CPS signal drops intermittently
- ECU loses track of crank position
- Engine management system cuts fuel/spark (safety)
- Engine stalls (or won’t restart)
Real example: Nissan Qashqai, 2015, 76,000 km. While driving: engine cuts out suddenly (stalls at traffic light). Restarts after 30 seconds. Happens twice more that day. Mechanic: CPS sensor intermittently failing (signal drops during acceleration). Replacement: sensor £135 + labor £85 = £220. After: no more stalling, smooth driving.
Cost to fix: £120–£250 Urgency: 🔴 CRITICAL (safety hazard, fix immediately)
Symptom #5: Poor Acceleration/Sluggish Response (Fifth — 10%)
What you notice:
- Engine feels weak when accelerating
- Delayed throttle response (engine takes time to respond)
- Feels like 20-30% power loss
- Especially noticeable: motorway merging (scary)
What’s happening:
- CPS signal weak/intermittent
- ECU can’t calculate precise timing
- Fuel injection/ignition timing suboptimal
- Less efficient combustion = less power
Real example: Honda Civic, 2013, 142,000 km. Owner accelerates to merge: car sluggish (delayed response). Feels unpredictable. Check engine light on (P0336 code). Mechanic: CPS sensor voltage unstable (bounces around instead of smooth signal). Replacement: sensor £105 + labor £75 = £180. After: immediate acceleration responsive again.
Cost to fix: £120–£250 Urgency: 🟠 HIGH (within 3 days)
Symptom #6: Engine Ticking/Knocking (Sixth — 5%)
What you hear:
- Metallic ticking/knocking from engine
- Gets worse during acceleration
- Sounds like engine pre-detonating (early spark knock)
- Usually accompanied by check engine light
What’s happening:
- CPS signal severely degraded
- Spark timing way off (either advanced or retarded)
- Advanced timing = pre-detonation (knock)
- Retarded timing = loss of power + knock
Real example: Mercedes C-Class, 2012, 167,000 km. Owner hears engine knocking (metallic pinging sound). Gets louder under acceleration. Brings to mechanic. Scanner: P0336 (crankshaft sensor performance) + knock detection codes. CPS signal completely unreliable. Replacement: sensor £160 + labor £95 = £255. After: knocking gone, smooth running.
Cost to fix: £120–£250 Urgency: 🟠 HIGH (within 3 days — engine damage risk)
Symptom #7: Limp Mode Engagement (Seventh — 2%)
What you notice:
- Check engine light + wrench symbol (limp mode active)
- Power severely limited (engine derated)
- Can only drive 25-30 mph maximum
- Can’t accelerate normally
What’s happening:
- ECU detects CPS failure completely
- Activates limp mode (safe fallback mode)
- Engine runs on fixed parameters (no optimization)
- Power cut to 50% for safety
Real example: Toyota Corolla, 2011, 154,000 km. While driving: check engine light + wrench symbol appears. Car suddenly derated to 25 mph (limp mode). Can’t drive normally. Called breakdown (£150). Tow to mechanic. Scanner: P0335 (crankshaft sensor circuit). Sensor completely dead. Replacement: sensor £115 + labor £80 = £195. Total with tow: £345.
Cost to fix: £120–£250 + tow £100–£150 Urgency: 🔴 CRITICAL (IMMEDIATE — don’t drive normally)
How to Diagnose — Professional Testing
Best diagnostic approach:
Test 1: OBD2 Scanner (First Step)
What to do:
- Connect OBD2 scanner to diagnostic port
- Read trouble codes
- Look for P0335, P0336, P0337
Results:
- Code present: CPS definitely involved
- No code but symptoms: Intermittent sensor failure (harder to diagnose)
Cost: £0–£50 (if mechanic does it)
Test 2: Voltage Output Test (Professional)
What to do:
- Mechanic connects multimeter to CPS connector
- Measures voltage while engine cranks
- Compares to spec (usually 0–5V varying pattern)
Results:
- Smooth voltage signal: Sensor OK
- Erratic/jumping: Sensor degraded
- Stuck voltage or no signal: Sensor dead
Cost: £40–£80
Test 3: Visual Inspection (Free)
What to do:
- Locate CPS (usually near crankshaft pulley)
- Check for: corrosion, loose connector, physical damage
- Check wiring for fraying/cuts
Results:
- Clean, tight: Connector OK
- Corroded: Clean connector, may solve problem
- Damaged: Must replace sensor
Cost: £0 (visual only)
Real Cost Breakdown — UK Pricing
| Service | Cost | Time | Difficulty |
|---|---|---|---|
| OBD2 scan | £0–50 | 15 min | Easy |
| Voltage test | £40–80 | 30 min | Professional |
| Visual inspection | £0 | 10 min | DIY |
| CPS part (OEM) | £80–150 | – | – |
| CPS part (aftermarket) | £50–100 | – | – |
| Labor (replacement) | £60–120 | 1–2 hours | Professional |
| Total replacement | £140–270 | 1–2 hours | – |
| Professional diagnosis | £60–100 | 30 min | Professional |
Prevention cost (cheapest):
- Annual inspection: £0 (visual)
- Preemptive replacement at 150,000 km: £140–200
Frequently Asked Questions
Can I drive with a bad crankshaft sensor?
No. If CPS fails completely: engine won’t start or stalls (undriveable). If degraded: may start but rough/unreliable. Don’t drive — replace immediately.
How much does crankshaft sensor replacement cost?
£140–£270 typically in UK. Sensor part: £80–150. Labor: £60–120. Varies by vehicle make/model.
Will CPS sensor code clear itself?
No. Once stored, code stays until cleared with scanner. Problem must be fixed first, then code clears and doesn’t return.
Is CPS covered by warranty?
Rarely. Sensor is wear item. Most warranties exclude after 3 years/60,000 km. Check your specific warranty.
Can bad CPS cause transmission problems?
Indirectly yes. Without accurate crank signal, ECU can’t calculate transmission shift timing properly. Transmission shifting rough if CPS bad.
What if I just clean the connector?
Sometimes cleaning corroded connector fixes problem (if corrosion was sole issue). Try cleaning first (free). If signal still erratic, sensor itself is dead (must replace).
Can I replace CPS myself?
Yes, if mechanically experienced. Usually accessible (1–3 bolts), simple electrical connector. Most DIY-friendly sensor on engine. Cost savings: £60–120 labor.
Why did my CPS fail?
Age (most common after 120,000+ km). Or: water intrusion (moisture damages electronics), corrosion from salt (winter driving), heat damage (engine bay extreme temps).
Related Information — Understanding Engine Management Sensors
When your engine won’t start, multiple sensors could be involved. Understanding which sensor failed helps target repairs. For fuel system sensor issues:
How to Know if My Fuel Pump is Bad
This explains fuel delivery sensor failures that mirror CPS symptoms (no start condition).
Additionally, when crankshaft sensor fails, fuel injectors suffer from wrong timing. For understanding fuel injection issues:
Signs of a Blocked Fuel Injector
Explains how CPS timing problems stress fuel system components.
For comprehensive sensor failure diagnosis:
What Sensors Can Cause Limp Mode
Crankshaft sensor is primary limp mode trigger — this guide explains all sensors that activate safety mode.
Prevention — Keep Your CPS Healthy
✅ Every 6 months:
- Listen for stalling (intermittent signal = warning)
- Note rough idle (misfires = sensor degrading)
- Check for check engine light (code P0335/P0336)
✅ Every 12 months:
- Professional CPS voltage test (preventive check)
- Inspect CPS connector for corrosion
- Check wiring for damage
✅ Every 100,000 km:
- CPS inspection (preventive assessment)
- Consider preemptive replacement at 150,000 km
✅ Driving habits:
- Don’t ignore stalling (dangerous + engine damage risk)
- Listen to engine sounds (rough/knock = CPS issue)
- Address issues within 1 week of noticing
According to engine management standards, crankshaft sensor is most critical engine sensor. Regular inspection prevents stranding and expensive damage.
Emergency Solutions — If CPS Failing But Can’t Repair Immediately
If check engine light (P0335) but you can’t repair now:
- Plan replacement immediately (don’t delay)
- Avoid long drives (stalling risk too high)
- Keep jump leads/breakdown cover handy (for stranding)
- Don’t ignore stalling (dangerous, fix this week)
- Drive locally only (not motorway)
If engine stalls: Emergency stop safely (hazard lights on), restart. Get to nearby garage immediately.
When to Seek Professional Help
Get professional help immediately if:
- Check engine light with P0335/P0336 codes
- Engine won’t start (cranks but no catch)
- Engine stalls while driving (safety hazard)
- Rough idle with misfires appearing
- Limp mode activated (can’t drive normally)
Professional replacement ensures: CPS mounted correctly, sensor aligned to crankshaft reluctor ring, wiring properly secured, timing verified after replacement.
Is your engine showing CPS failure signs? Tell me symptoms (won’t start? rough idle? stalling?) — I’ll tell you exact sensor status and replacement urgency.
