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Rough Idling & Stalling in 2016 CLA250 - Suspect CPS

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Mike T. · rep 0 · May 25, 3:07 AM · 1 replies · solved · Question

Vehicle: 2016 Mercedes-Benz CLA250 Sedan

Hey everyone,

I'm having some concerning issues with my 2016 Mercedes-Benz CLA250 Sedan. It's developed a really rough idle, almost like it's struggling to stay on. Sometimes, especially after driving for a bit and then coming to a stop, it will just stall completely. It also feels a bit hesitant to start, particularly when the engine is warm.

The check engine light is on, but I haven't had a chance to get the codes read yet. I've read online that these symptoms can point to a failing crankshaft position sensor (CPS) in Mercedes models. My car has about 85,000 miles.

Does this sound like a CPS issue to you all? Is it a difficult DIY fix, or should I definitely take it to a shop? Thanks for your help!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 3:07 AM

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ClearTheCode admin answer:

The symptoms you're describing for your 2016 Mercedes-Benz CLA250 Sedan – rough idling, stalling (especially when warm), and difficulty starting – are indeed classic indicators of a failing crankshaft position sensor (CPS).

Here's why and what to do:

  1. Crankshaft Position Sensor Function: The CPS is a critical engine management sensor. It monitors the rotational speed and position of the crankshaft and sends this information to the engine control unit (ECU). The ECU uses this data to precisely time fuel injection and ignition. If the sensor fails, the ECU loses this vital information, leading to severe engine performance issues.

  2. Why Symptoms Worsen When Warm: CPS sensors are often inductive sensors, and their internal components can become more susceptible to electrical resistance or signal degradation when exposed to heat. This is why the problem often manifests or worsens once the engine reaches operating temperature.

  3. Common OBD-II Codes: When a CPS fails, the check engine light will illuminate, and you'll almost certainly find a diagnostic trouble code (DTC) related to the crankshaft position sensor circuit. The most common code is P0335: Crankshaft Position Sensor 'A' Circuit Malfunction. Other related codes might appear depending on the specific failure mode.

  4. Diagnosis and Action:

    • Get Codes Read: Before doing anything, get the OBD-II codes read. This will confirm if P0335 or a similar code is present, strongly pointing to the CPS.
    • Visual Inspection: While difficult to see without lifting the car, a visual inspection of the sensor and its wiring for any obvious damage or corrosion is a good first step.
    • Replacement: Replacing the CPS can be a DIY job for some, but it depends on its location on your CLA250's engine. It's typically located on the engine block near the flywheel or transmission bell housing. Access can be tight, requiring specific tools and potentially working from underneath the vehicle. Ensure you use a high-quality OEM or equivalent replacement part.

  5. When to See a Shop:

    • Access Difficulty: If you're uncomfortable working in tight spaces or don't have the necessary tools (e.g., jack stands, specific wrenches), a professional shop is recommended.
    • Confirmation: If the codes are not P0335 or directly related to the CPS, or if replacing the sensor doesn't resolve the issue, a professional diagnosis is essential to rule out other potential causes like fuel delivery problems, ignition system issues, or other sensor failures.
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