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Smart Fortwo Camshaft Position Sensor Replacement — 451 and 453

Moderate 45-75 min $40-110Smart Fortwo 451Smart Fortwo 453Smart Forfour 453

Tools you'll need

  • OBD2 scanner that reads live data (not just code-clear)
  • 8mm or 10mm socket for the sensor hold-down bolt (varies by year)
  • 1/4" ratchet and short extension
  • Torque wrench (0-20 Nm range)
  • Pick or small flat screwdriver to release the connector clip
  • Clean shop rag, brake cleaner for the sensor seat
  • New O-ring (usually included with OEM sensor; verify before install)

What this is + why it matters

The camshaft position sensor (CMP) tells the ECU which cylinder is on the firing stroke versus the exhaust stroke. The crank sensor alone can't tell those apart on a four-stroke — it sees the same crank position twice per cycle, once compressing and once exhausting. The cam sensor resolves that ambiguity so the ECU can fire the right plug at the right time and inject fuel into the right cylinder.

When the CMP fails, you usually get one of four codes: P0340 (circuit), P0341 (range/performance), P0345 or P0346 (similar, on the secondary cam if your variant has VVT). Symptoms run from longer cranking before start, to a slight stumble until the ECU figures out where it is, to a no-start if the failure is total. Some Smarts will fall back to "batched fire" mode where the ECU sprays fuel and fires all plugs every revolution — the engine runs but it's down on power and burning extra fuel. Either way, you'll have a check engine light and the car will feel off.

In the real world, most CMP-coded Smarts come in with one of three stories. Story one: cranks for three or four seconds before catching, then runs fine — that's an intermittent signal at startup, often an O-ring leak filling the connector with oil. Story two: random check engine light with a slight stumble at low rpm — that's a sensor on the way out but still mostly working. Story three: dead no-start with the security light flashing — that's a total signal loss, and the ECU has refused to fuel without knowing where the cam is.

It's a cheap part and a quick swap if you've actually identified the right failure. The trap is that several other things mimic CMP failure on these engines — that's where most of the wasted money goes.

What you'll need

Tools and parts above. A few specifics:

  • OEM Bosch or Mercedes-spec sensor for the 451 M132. OEM Renault-spec for the 453 H4Bt. Generic eBay sensors are the leading cause of "I replaced the CMP and the code came back two weeks later." Don't skip the OE part on a $40 component.
  • A new O-ring at the sensor base. Most OEM sensors ship with one; cheap aftermarket often don't. The old O-ring is hard, deformed, and often the actual root cause — see Common gotchas.
  • A scan tool that reads live data, not just one that clears codes. You want to see cam-to-crank correlation in real time before and after the swap. The cheapest Bluetooth OBD2 dongle plus Torque Pro or Car Scanner on a phone covers this.
  • Electrical contact cleaner. A $5 can. Worth every cent — most "failed" cam sensors are just contaminated connectors, and you find that out by cleaning the pins before you order anything.
  • A magnet on a stick or a borescope if you're nervous about dropping the bolt. The 453 in particular has the engine right under you and a dropped 8mm bolt can ride a casting all the way to the bottom of the bay.

Step by step

Diagnose first, replace second

  1. Read the codes properly. P0335/P0336 are crank sensor codes — different sensor, different failure, different fix. P0340/P0341/P0345/P0346 are cam codes. If you see both crank and cam codes set at the same time, suspect the timing belt or chain has jumped a tooth and stop here — replacing sensors won't fix mechanical timing.
  2. Pull the connector and look at the pins. Before you order anything. Oil in the connector means your sensor probably isn't dead — the O-ring is leaking and shorting the signal pins. Clean the connector with electrical contact cleaner, dry it, run the car, see if the code comes back. If it does, then order the sensor.
  3. Check live data if you have a scan tool that supports it. Cam-to-crank correlation should be steady at idle. If it's drifting in and out of spec, that's your sensor (or wiring). If it's clean, look elsewhere — start with the misfire codes, the spark plugs, and on the 453 the valve cover gasket.

451 (M132 petrol)

  1. Engine cool, ignition off, key out. The sensor lives in the head and the area gets hot.
  2. Open the rear hatch and pull the plastic engine cover. Twist or unclip the fasteners depending on year.
  3. Find the CMP on the cylinder head or valve cover. It sits with its body sticking up out of the head and the connector facing roughly forward. Single mounting bolt — usually 8mm or 10mm, occasionally Torx on later years.
  4. Disconnect the harness. Press the locking clip with a fingernail or pick, pull the connector straight back. Don't yank the wire.
  5. Unbolt and pull the sensor straight out. No twist. If it's stuck from baked-on oil or sealant, wiggle it gently in line with the bore. A positioning tab will shear if you rotate it.
  6. Inspect the bore and the old O-ring. Wipe out any oil with a clean rag. If the old ring is hard, cracked, or coated in baked-on residue, that was likely your problem.
  7. Fit the new O-ring (light smear of fresh engine oil on the ring helps it seat) and push the new sensor home by hand. It should seat fully without forcing.
  8. Hand-thread the bolt, then torque. Around 8-10 Nm typical for the M132 — verify your year in the workshop manual. Snug, not gorilla.
  9. Reconnect the harness, listen for the click, replace the engine cover.

453 (H4Bt 0.9L turbo or B4D 1.0L NA)

  1. Engine cool, cover off the engine bay. Same starting point.
  2. Locate the sensor on top of the engine. On the H4Bt the CMP is typically accessible from above with the cover off. The connector usually faces the rear of the engine. On some builds a wiring loom bracket sits in front of it — pop the bracket aside if it's blocking the bolt.
  3. Disconnect the harness, unbolt the sensor, pull straight out. Same care as the 451 — no twist.
  4. Wipe the bore. If oil is pooling around the sensor seat and you have a 453, suspect the valve cover gasket before condemning the sensor. See gotchas.
  5. Fit the new O-ring on the new Renault-spec sensor and seat it. Hand-thread the bolt, torque to spec, reconnect the harness.

After replacement

  1. Clear the codes (or let them self-clear over one to two drive cycles for pending codes; confirmed codes usually need an active scan-tool clear).
  2. Start it, idle for a minute, listen. No new ticks, no rough idle.
  3. Drive normally. Watch for the code returning. If it comes back inside a week, the sensor wasn't the root cause — go back to step 1 and look at wiring, oil contamination, or timing.

Common gotchas

  • Oil in the connector is the most common false-fail. A leaking CMP O-ring lets oil track up the sensor body and pool around the pins. The signal goes intermittent and the ECU sets a cam code. People throw a sensor at it; the new sensor's new O-ring fixes the problem and the sensor itself was fine. Always inspect the connector before ordering parts. Clean the pins with electrical contact cleaner, replace the O-ring, and see if the code comes back before condemning the sensor.
  • CMP and CKP codes get confused constantly. P0335/P0336 = crank position sensor. P0340/P0341/P0345/P0346 = cam position sensor. Different parts, different locations, different failure modes. Don't assume — read the code first. The crank sensor swap is on its own page.
  • 453 valve cover gasket leak masquerades as a CMP failure. The H4Bt has a well-known weeping valve cover gasket. When it leaks, oil pools around the cam sensor area and seeps into the connector. Symptoms look like CMP failure (P0340, P0303, sometimes both) but the fix is a new valve cover gasket, not a new sensor. There's a dedicated fault code page on this site for the P0303 oil-fouled variant — start there if you're a 453 owner and you see oil anywhere near the sensor.
  • Cheap aftermarket sensors fail fast. A $12 sensor off a marketplace will often work for a few weeks then set the same code again. The Hall-effect element inside is the part that fails on the cheap ones. OEM Bosch on the 451 and OEM Renault-spec on the 453 are the right call.
  • Don't twist on removal. The sensor body has a tang or alignment feature on some variants. Pull straight out, no rotation, or you'll snap plastic into the head. If you do snap it, retrieve every fragment with a magnet and a borescope before installing the new sensor — anything left in the head ends up in the cam area.
  • A misfire-looking symptom can actually be the cam sensor. A flaky CMP can drop fuel injection on one cylinder for a beat or two until the ECU resyncs. That looks identical to a P0301/P0302 misfire on a scan tool, and people swap plugs and coils chasing it. If you have a misfire code with no obvious cause and the plugs and coils are recent, scope the cam sensor before buying more ignition parts.
  • Some codes won't clear on their own. P0340 stored as a confirmed code may need an active clear with a scan tool. Driving it out works for pending codes but not always for confirmed ones.
  • P0345/P0346 on the M132 Brabus usually points at the exhaust-cam sensor or the VVT solenoid, not the intake-cam sensor. If your code is on bank 2 / sensor B and you replace the intake CMP, you've fixed the wrong part. Read the freeze-frame data and confirm which sensor the code is naming before you order.
  • Don't pinch the wiring loom on reinstall. Easy to do on the 453 if you reroute the harness around a bracket. A pinched wire chafes through within a few months and gives you the same code back, except now it's intermittent and harder to find.
  • Don't reuse the bolt if it's a one-time-torque-to-yield design. Most years use a normal reusable bolt, but a few late-453 builds spec a torque-to-yield fastener for the sensor mount. Workshop manual will say. If yours does, throw the old bolt out and use a new one.

When to skip DIY

If the CMP is in a tight spot on your specific year — a few 453 builds tuck the sensor behind brackets that have to come off first — and you don't have a torque wrench, take it to a Smart-experienced shop. Expect $120-220 with parts and labour on a straightforward swap; less on a 451, more on a 453 if the loom or bracket is in the way.

The other reason to outsource is if you've already replaced the sensor once and the code came back. At that point the issue is a wiring fault, an oil leak feeding the connector, or a timing chain problem, and a shop with a proper scope will find it faster than you will at the kerb. The same applies if you have both a cam code and a crank code together — that's a "stop guessing, get it on a scope" situation, not a parts-cannon situation. Throwing a $40 sensor at a $400 timing problem buys you nothing.

Diesel CDI owners — your engine doesn't have a CMP in the same role; it has a different sensor on the high-pressure pump and a different family of codes. This page doesn't cover that.

Manual references

Top reference manuals for this chassis (from our catalog of 88 Smart manuals):

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