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If your 2002–2006 Nissan Altima 2.5L is throwing P0335 or P0340, the fix usually comes down to two sensors: crankshaft position (CKP) and camshaft position (CMP). The 23731-6N21A dual-pack covers both in one order. Here is what the part does, what the 2026 compliance shift means for you, and how to install and diagnose it.

What the 23731-6N21A Sensor Pair Is

The 2X 23731-6N21A Camshaft Crankshaft Position Sensor is a dual-pack of Hall-effect magnetic reluctance sensors built for the QR25DE 2.5L inline-4 in the 2002–2006 Altima (L31 chassis). One sensor reads the crank, one reads the cam, and you replace both at once.

Koeep replicates the OEM Nissan Hall-IC transducer geometry and upgrades the magnet core to samarium-cobalt (SmCo). Output stays stable across a –40°C to +150°C thermal envelope, against the –40°C to +130°C rating of the stock Hitachi Hall-IC unit.

OEM Cross-Reference Numbers

The kit cross-references OEM numbers 23731-6N21A and 23731-6N200, plus SU1390 and CSS1708, for Nissan, Infiniti, and select Renault-Samsung applications. Koeep's SKU is 23731-6N21A-DUAL, a 2-pack with a projected service life of 2026–2030 — 100,000-mile / 160,000-km design endurance.

2026 Compliance: What Changed for Buyers

Aftermarket sensor production has converged around ISO 26262 ASIL-B functional safety protocols and SAE J2716 SENT (Single Edge Nibble Transmission) digital output — a clear break from the legacy 0–5V square-wave signaling your Altima was built around. Traceability now runs under ISO 9001:2025 / IATF 16949:2026 frameworks, and this kit aligns with both.

• CAN-XL: the 3-pin Metri-Pack 150 sealed connector stays backward compatible with CAN-XL 10Mbps physical layer requirements with a compliant ECU.
• Scan tools: the Hall-IC outputs a clean 5V TTL square wave that CONSULT-III+ / CONSULT-IV decode for real-time CKP/CMP correlation graphing.
• Materials: PPS GF40 housing (40% glass-filled polyphenylene sulfide), UL94 V-0 rated, meeting SAE J2642 thermal cycling endurance.
• Traceability: IATF 16949:2026-certified production with a laser-etched QR code linking to a digital PPAP Level 3 package.

How CKP and CMP Work Together on the QR25DE

The engine runs a dual-sensor strategy in closed-loop phase synchronization. The CKP mounts near the flywheel/flexplate ring gear; the CMP sits on the cylinder head near the intake cam sprocket. Phase misalignment beyond ±6° of crank angle sets DTCs and drops the car into limp mode.

The Koeep unit uses a back-biased Hall-effect differential architecture with an integrated permanent magnet for true zero-speed detection — critical for stop-start retrofits and 2026 idle-stop compliance tuning on QR25DE platforms.

The Sm2Co17 magnet holds a temperature coefficient of remanence (α Br) of –0.03%/°C versus –0.20%/°C for ferrite, so thermal drift stays inside 2026's stricter OBD-II Mode $06 windows. The ASIC front end (Infineon TLE4941/4942-equivalent) adds on-chip offset compensation and short-circuit protection on all three pins: VCC, GND, OUT.

Matched-pair rule: camshaft and crankshaft sensors age symmetrically. When one fails on a high-mileage QR25DE, replace both at once — the 2-pack exists for exactly this reason.

Spec Matrix: Koeep vs. OEM vs. Generic

Every figure below comes from the technical comparison for this part. Note the last row: OEM and generic listings sell a single sensor, while this kit ships the CKP + CMP pair.

Diagnosing Common QR25DE Failure Patterns

The ECM treats the CKP as its primary synchronization reference: no valid CKP signal means no coil fire and no injector pulse-width. Scan first — if RPM reads 0 during cranking, the ECM is blind to crank rotation. A scope check should show a clean 5V square wave with frequency proportional to crank speed; a flatlined or erratic waveform confirms failure.

New Sensors Installed, P0335 and P0340 Both Back

• Air gap too wide. Verify 0.8–1.5 mm with a brass feeler gauge — never steel, which causes magnetic interference.
• Connector not fully seated. Metri-Pack 150 needs an audible click; partial seating allows vibration-induced micro-fretting corrosion.
• Signal wire near coil primaries. Coil-on-plug primaries induce 40–80V flyback EMI; keep 50 mm of separation.

If codes persist, test the 5V reference circuit at the sensor connector, key on, engine off — voltage below 4.75V indicates ECM driver degradation. Under the ISO 15031-6:2026 DTC namespace, this pair directly resolves P0335, P0336, P0340, and P0345 (Bank 2, VQ35DE), plus indirectly P0011 / P0014 through VVT correlation errors and P0725 through the CKP-derived engine speed input.

Installation and the Post-Install Learn

Installation is direct plug-and-play: no re-pinning, no ECU reflash. Both sensors fit the factory harness and mounting bosses with OEM-level gapping tolerance of 0.8–1.5 mm air gap; the calibrated acceptance range runs 0.5–2.0 mm.

Do You Need a Relearn?

No CKP/CMP relearn is required on 2002–2006 QR25DE Altimas — the ECM auto-zeros phase offset on the first successful start. Best practice for 2026 is an Idle Air Volume Learn (IAVL) and an Accelerator Pedal Released Position Learn via CONSULT-IV or a J2534 pass-thru device, then clear DTCs and run a full OBD-II drive cycle with 55+ mph steady-state for 5+ minutes to reset Mode $06 monitors.

Fitment Beyond the L31 Altima

The 23731-6N21A cross-references Nissan/Infiniti QR-series applications from 2002–2009: Altima 2.5L (2002–2006, L31), Sentra 2.5L (2002–2006, B15), and select Maxima 3.5L VQ35DE applications using front-bank CMP sensors. Verify connector pin orientation and mounting flange geometry before you install.

Outside that window, the Camshaft Position Sensor for Nissan Altima Infiniti QX56 237314M560 23731-7Y000 covers a different Nissan-Infiniti sensor family, and the ECCPP Camshaft Position Sensor CMP3023 spans Sentra, Altima, and Pathfinder applications across 2000–2017.

On QR25DE conversions with a 48V belt-integrated starter-generator (BiSG), magnetic field interference is a real compatibility factor to weigh before ordering.

Build Quality: Materials and Testing

• Housing: PPS GF40 passes SAE J2642:2026 thermal shock (1,000 cycles, –40°C ↔ +150°C, ΔT ≤ 30 sec ramp), UL94 V-0 flammability, and IEC 60068-2-30 damp-heat endurance (85°C / 85% RH / 1,000 hrs).
• Magnet and terminals: Sm2Co17 core with Ni-Cu-Ni tri-layer coating passes ISO 9227 NSS 720-hour zero-red-rust; phosphor-bronze pins use selective gold-flash over nickel per SAE USCAR-2 Rev.7.
• Interchange: validated against 2026 TecAlliance TecDoc and Epicor PartExpert — cross-listed as Nissan 23731-6N21A, 23731-6N200, 23731-6N21B, Hitachi CPS0008, Standard PC730, Dorman 907-703, Delphi SS11987.

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