Home Maintenance Electrical

Smart Fortwo Crankshaft Position Sensor Replacement — 451 and 453

Moderate 60-90 min — access dependent $35-110Smart Fortwo 451Smart Fortwo 453Smart Forfour 453

Tools you'll need

  • 8mm and 10mm sockets, short and long extensions
  • 3/8" ratchet
  • Torque wrench (low range, 5-25 Nm)
  • Jack and jack stands (or ramps) — most years need underside access
  • Wheel chocks
  • Pick or small flathead for the sensor connector clip
  • Electrical contact cleaner
  • Dielectric grease for the connector
  • OBD scan tool — to confirm the code before parts and to clear it after
  • Shop light, magnetic pick-up tool (the bolt likes to fall)

What this is + why it matters

This guide covers the petrol variants only — 451 M132 (NA and Brabus) and 453 H4Bt (turbo) / B4D (NA). The 451 CDI diesel uses a different position-sensing setup tied to its fuel injection system; the diagnostic and replacement path is not the same. If you have a CDI, this is not your guide.

The crankshaft position sensor (CKP) tells the ECU exactly where the crank is in its rotation, by reading the teeth on the flywheel reluctor ring. Without a clean signal the ECU has no reference for spark timing or fuel injection, and the engine either won't start, stalls intermittently when hot, or runs rough enough to throw a misfire fog. CKP failures are common past 100,000 miles on small high-revving engines like the M132 in the 451 and the H4Bt in the 453.

There are two classic failure patterns, and they fool people into replacing the wrong parts.

The first is heat-soak: the car drives fine, you shut it off in a parking lot, and ten minutes later it cranks normally but won't fire. Wait an hour, it starts. Wait three hours, it starts. The sensor's internal windings open up as they heat through soak, then close again as they cool. From a driver's seat it feels random. From a wiring diagram it's textbook.

The second is random stall in traffic, usually with a P0336 logged. The car cuts out at idle or low load, sometimes restarts on the next crank, sometimes coasts to the shoulder before it'll fire again. Both patterns cost owners new starters, new batteries, and new fuel pumps before someone runs an OBD scan and finds P0335 or P0336 sitting in memory. Don't be that owner. The fix is a $40-90 sensor and an hour of access work.

The CKP and the camshaft position sensor (CMP) are siblings — same family of failure mode, similar replacement procedure, different location. If the OBD scan brings up P0335/P0336 and a P0340/P0341, look at both sensors. Same heat-soak failure pattern. Same cheap-first diagnostic order. There's a separate camshaft-sensor-replacement page on this site for that one.

What you'll need

The full tool list is in the table above. A few specifics worth calling out:

  • An OBD scan tool is mandatory before you buy parts. The symptoms overlap with at least four other failure modes — bad battery, weak starter, fuel pump, ignition coil. You're looking for P0335 (no signal at all) or P0336 (signal present but out of range). Different fault, same sensor, sometimes a different problem under the surface.
  • Buy OEM Bosch or OEM-spec. Cheap aftermarket CKPs are notorious for failing inside a year, and the symptom on failure is the same one you're trying to fix. The $30 you save up front is a $30 fee for doing the job twice.
  • A magnetic pick-up tool earns its keep here. The single retaining bolt sits in a spot where if it falls, you're fishing it out of the bellhousing area or the underbody shielding. Plan for it.
  • Electrical contact cleaner and dielectric grease. Half the "failed sensor" diagnoses on these cars are corroded connectors. Have both on hand and clean the connector before you condemn the sensor itself.
  • A second person to crank the engine if you want to scope the signal. Optional but useful on a stubborn intermittent — an oscilloscope on the sensor leads while a helper cranks shows you whether the signal is clean, weak, or missing teeth. If you don't have a scope, skip it and let the OBD code do the talking.

Step by step

The procedure is the same shape on the 451 M132 and the 453 H4Bt — the sensor lives near the bellhousing reading off the flywheel teeth in both. What varies is access. Some years and trims you can reach it from above with the airbox and intake out; on most you'll be on your back from underneath. Crawl under and eyeball the connector run before you commit to a route, and verify the location in your year's workshop manual — sensor placement moved at least once across the M132 production run.

  1. Confirm the code first. OBD scan, write down P0335 or P0336 (sometimes both), then clear it and see if it returns on a hot drive. If you can reproduce the heat-soak no-start pattern, that's near-conclusive.
  2. Cool the engine down. The sensor sits against hot metal. Working it out hot risks burns and risks deforming the new sensor's O-ring as you push it home.
  3. Disconnect the negative battery terminal. The CKP feeds the ECU directly. You don't want a stray ground while you have the connector apart, and the ECU likes to clear adaptive memory after a battery disconnect — useful here since you want it relearning.
  4. Lift and secure the car if you're going from underneath. Jack stands on rated lift points, wheels chocked. Never crawl under a Smart on a jack alone.
  5. Locate the sensor at the bellhousing. It's a small cylindrical body, usually with a single bolt holding it in a bracket and a 2- or 3-pin connector running off it. On the 451 M132 it sits on the engine block near where the engine meets the transmission, reading the flywheel teeth through a small bore. On the 453 H4Bt it's in a similar position on the H4Bt's bellhousing area.
  6. Disconnect the connector first, and inspect it carefully. Press the locking tab, pull straight off — no rocking. Then look. Green crust on the pins, oily film, a pin pushed back in the housing, water in the cavity — any of those explain the symptoms before you even get to the sensor. Hit it with electrical contact cleaner, work it dry with compressed air or a clean rag, reseat the connector, and try the car. A surprising number of "failed CKP" complaints end here for a few dollars in cleaner. Only if the connector is clean and the symptom returns do you commit to a new sensor.
  7. Remove the retaining bolt. Usually 8mm or 10mm. Magnetic socket recommended — if it falls, it falls into a place you don't want to be reaching for it.
  8. Pull the sensor straight out. It's a snug interference fit on the O-ring. Wiggle gently and pull. Don't pry against the bracket. If it fights you, twist a quarter-turn each way to break the seal.
  9. Inspect the old sensor and the bore. A tip glazed with oil means a rear main seal weep is contaminating the sensor — that's a bigger conversation. A clean tip and a dry bore is the normal case.
  10. Lubricate the new sensor's O-ring with a film of clean engine oil. Don't run it dry into the bore; you'll shear the O-ring.
  11. Seat the new sensor. Push it home until the bracket meets the mounting surface flush. Most CKPs are self-spacing — the snout touches the flywheel teeth on rotation, springs back to its plastic stop, and sets the air gap automatically the first time you crank the engine. You don't need a feeler gauge. What you do need is the bracket sitting flat against its mounting boss with no gap; if there's a sliver of light between the two, you have something pinched (usually the wiring loom or a piece of debris in the bore).
  12. Snug the retaining bolt to spec — do not over-torque. Typically 5-10 Nm; verify in the workshop manual for your year. Over-torquing cracks the sensor body or pulls the bracket out of square and you'll be back here in a month chasing the same code on a brand-new sensor.
  13. Reconnect the harness. A pinhead of dielectric grease in the connector cavity keeps moisture out. Listen for the click as the locking tab seats — if you don't hear it, the connector isn't fully home, and you'll be back at step 6 in a few weeks.
  14. Reconnect the battery, clear the code with your scan tool, start the engine. It may idle rough or hunt for thirty seconds while the ECU re-references. If it idles smooth and stays running, drive it.
  15. Drive cycle for relearn. The ECU may need a few minutes of mixed-load driving — varying rpm, light acceleration, decel — to fully relearn its position reference. Some bidirectional scan tools can force a CKP relearn faster, which is useful if you need to re-scan immediately. Either way, the code shouldn't return; if it does, suspect the connector, the wiring run from sensor to ECU, or a tone-ring problem on the flywheel.

Access notes by chassis

451 M132 (petrol / Brabus). Most years let you reach the sensor from underneath with the car on stands. Some early 451s allow top access with the airbox lifted out — useful if you can't get a car off the ground. The connector is usually routed along the engine harness toward the rear of the bay; trace it back to confirm you've got the right sensor before you start unbolting. If you're under the car, drop the bottom engine cover (a few plastic fasteners) for working room.

453 H4Bt (0.9L turbo) and B4D (1.0L NA). Tighter access, especially on the H4Bt. The Renault-platform layout puts more accessory plumbing in the way. Plan on intake snorkel removal at minimum. Allow more time than the 451; the actual swap is the same shape but the path to it is longer.

Common gotchas

The cheap-first order on a CKP fault is: scan, then connector, then sensor, then wiring, then tone ring. Most jobs end at the connector or the sensor. Owners who skip ahead lose money and time.

  • Heat-soak no-start misdiagnosed as starter, battery, or fuel pump. A car that won't restart hot but cranks normally and starts fine cold is the textbook CKP heat-soak failure. Run an OBD scan before throwing a $200 starter at it. The starter that "fixed" it for a week probably just gave the sensor enough cool-down time to start working again.
  • Hot-engine refusal where the engine WILL start cold = classic CKP. Don't tow it. Pop the hood, wait an hour, and it'll start. That's the diagnosis confirming itself.
  • Chasing the wrong code. P0335 means the ECU sees no CKP signal at all — usually a connector issue or a fully dead sensor. P0336 means there's a signal but it's out of expected range — that can be a heat-failing sensor, a damaged tone ring on the flywheel, or wiring chafe. They look similar from the driver's seat; the diagnostic path is different.
  • Cheap aftermarket sensors that fail within a year. OEM Bosch or known-good OEM-spec is the only sensible buy. The $30 generic on a marketplace listing is the second-most-common reason for a re-do. If the price looks too good, it is.
  • Counter-staff at chain parts stores hand you the wrong sensor. The 451 M132 and the 453 H4Bt do not share a CKP. Match the part number against your year and engine before you walk out, not after.
  • Oil leak from the rear main can soak the CKP. A rear main seal weep dribbles onto the sensor and the bore, and the contaminated tip throws the same codes a failing sensor does. Clean the area before condemning the sensor — and if the bore stays oily, you have a bigger leak to address first that no new sensor will solve.
  • Damaged flywheel tone ring mimics sensor failure. Rare but real, especially after starter motor work or a dropped flywheel during clutch service. If you're on your second sensor in two months and the codes keep coming back, get a scope on the signal — the tooth pattern will tell you whether the ring or the sensor is at fault. Tone ring damage is a transmission-out repair; that's where DIY ends.
  • CDI diesel owners — this is not your sensor. The 451 CDI uses a different position-sensing setup tied to the diesel injection system. P0335 on a CDI follows a different diagnostic path. This guide is petrol only.
  • Don't over-torque the retaining bolt. It's a small bolt into a bracket. Snug, not heroic. Cracking the sensor body on installation is the third-most-common reason for a re-do.
  • Symptom comes back after a week of clean running. That usually means a connector issue you didn't fully clean, a wiring chafe upstream of the sensor, or — rarely — an aftermarket sensor that's already failing. Re-scan first; the new code (or absence of one) tells you which it is.

When to skip DIY

If the sensor sits on a year and trim where you can reach it from the top with the airbox out, this is a confident-DIY job for anyone who can read a scan tool, work clean, and trust a torque wrench. The only special tool is the OBD scanner, and most owners already have one.

If yours is buried — some 453s especially — and the access requires intake removal or accessory disassembly, the labour starts to outpace what a Smart-experienced shop will charge for the same job on a lift in half the time. Expect $200-350 at a shop including diagnosis, parts, and labour.

Also skip DIY if your car is still inside its powertrain warranty: P0336 in particular is often a covered repair on later 453s. Don't pay out of pocket for something the dealer is on the hook for.

If the symptoms persist after a fresh sensor, a clean connector, and a verified scan — that's tone-ring or wiring territory. Tone-ring repair means dropping the transmission to inspect the flywheel teeth. Wiring chafe means tracing the harness back to the ECU connector with a multimeter. Both are shop jobs for almost everyone.

One last note on parts pricing. An OEM Bosch CKP is usually $50-90 depending on application; aftermarket can dip to $25 but the failure rate isn't worth it. Dealer pricing on the same Bosch part is often double the parts-store price for the identical box. Cross-reference the OEM part number before you buy.

Manual references

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

Need something specific? Browse all 88 manuals by chassis, year, region, or document type.

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