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Check Engine Light On and AC Not Working: Mercedes-Benz CLS Case Study
When the check engine light comes on, most drivers expect an engine issue. But what if the air conditioning system stops working at the same time? That’s exactly what happened in this case study on a Mercedes-Benz CLS W257 with the M264 engine.
The problem turned out to be a broken wire on the outside temperature sensor, a small component with a surprisingly big role. This article will walk you through the diagnosis, repair process, and lessons learned, so you’ll know what to check if you ever face the dreaded combination of Check Engine Light On and AC Not Working.
Vehicle Overview
- Model: Mercedes-Benz CLS W257
- Engine: M264 petrol engine
- Customer Complaint: Check engine light on + AC not blowing cold air
At first glance, the driver expected two separate issues. But the diagnostic process revealed that both problems were linked to a single faulty sensor wire.
Step 1: Diagnostic Process
Quick Test with XENTRY
The technician connected the vehicle to the Mercedes-Benz STAR diagnostic system. The scan revealed:
- DTC P007312 – Outside temperature sensor has a short circuit to positive
This pointed directly to the ambient temperature sensor, which is responsible for:
- – Sending external temperature data to the engine control unit (ECU)
- – Providing data to the HVAC system for air conditioning operation
Step 2: Visual Inspection & Sensor Location
1. Wiring Diagram Check
The wiring diagram showed that the outside temperature sensor was located beneath the front bumper.
2. Physical Examination
- – Upon inspection, one of the wires was broken.
- – Likely causes: environmental wear, debris impact, or rodent activity.
Step 3: Repair Process
Repair Actions Taken
- 1. Wire Repair: Damaged wire was stripped, soldered, and protected using heat-shrink tubing for durability.
- 2. Code Clearance: Fault codes were cleared from the ME control unit using STAR diagnostics.
- 3. System Check: Both ECU and HVAC systems showed normal operation.
- 4. AC Test: Air conditioning resumed cooling function.
- 5. Warning Reset: Check engine light turned off.
Result: Both problems resolved with a simple wire repair.
Why a Broken Sensor Wire Affects the AC System
| System | Role of Outside Temp Sensor | Impact if Faulty |
|---|---|---|
| Engine (ECU) | Adjusts ignition timing & fuel trims based on outside air temp | Triggers CEL when values are out of range |
| AC (HVAC) | Uses temperature data to engage/disengage compressor | Prevents AC from turning on as a safety measure |
Without accurate outside temperature data, the AC system won’t activate the compressor, and the ECU flags an error.
Lessons for Car Owners & Technicians
- 1. Don’t jump to expensive parts. A broken sensor wire can mimic compressor or ECU failure.
- 2. Start with diagnostics. Always run a scan before replacing AC or engine components.
- 3. Check wiring early. Wiring damage is more common than people think, especially in exposed areas.
- 4. Cross-system dependencies matter. A single sensor feeds both the engine and climate control, so one fault can trigger multiple warnings.
Symptom – Cause – Fix Table
| Symptom | Likely Cause | Repair Solution |
|---|---|---|
| CEL + AC not working | Broken outside temp sensor wire | Repair or replace wiring |
| CEL only | Faulty sensor or short circuit | Replace sensor |
| AC not blowing cold | Blocked refrigerant flow or sensor fault | Inspect AC circuit + sensor |
| Flashing CEL + poor AC | Misfire affecting load on compressor | Diagnose ignition system |
Explore More Mercedes AC & Check Engine Light Guides
For a full breakdown of common cooling and climate control problems, visit our Mercedes AC and Climate Control Problems – Complete Troubleshooting Guide.
To understand every possible reason behind a dashboard warning, check out our Mercedes Check Engine Light – Complete Troubleshooting Hub
FAQs: Check Engine Light On and AC Not Working
Q: Can AC trigger a check engine light?
Yes. The AC system shares data with the ECU. If a sensor like the outside temp sensor fails, it can trigger the CEL.
Q: Can AC problems affect the engine?
Indirectly. A failing compressor can overload the serpentine belt, reducing performance and causing engine warning lights.
Q: Can low coolant cause AC failure?
Yes. If coolant is too low, the ECU may disable the AC compressor to protect the engine from overheating.
Q: Can a bad AC compressor cause engine problems?
Definitely. A seized compressor can cause belt slippage, overheating, or stalling. It may even trigger fault codes.
Preventive Tips
- – Inspect wiring harnesses during regular service.
- – Protect sensor wires in exposed areas with insulation or protective sleeves.
- – Use rodent deterrents if parking in environments prone to infestations.
- – Diagnose before replacing – avoid unnecessary compressor or ECU replacements.
Conclusion
In this Mercedes-Benz CLS W257 case study, both the check engine light and AC not working were caused by a broken wire on the outside temperature sensor.
A systematic approach scanning fault codes, inspecting wiring, and repairing instead of replacing expensive parts restored both systems to normal.
Author Bio
Written by Mercedes Expert
With years of hands-on experience diagnosing and repairing Mercedes-Benz systems, he brings technical depth and practical case studies to help car owners, technicians, and enthusiasts troubleshoot complex automotive issues. His work focuses on clear repair guides, OEM-level procedures, and knowledge-sharing to empower both professionals and drivers.
Last update: September 2025
