Mercedes SLK Common Problems: R170 and R171 Guide

The Mercedes-Benz SLK combines open-air driving with luxury, but these roadsters have well-documented issues. Learn what to watch for and how to fix them.

The Mercedes SLK (Sportlich, Leicht, Kurz - Sporty, Light, Short) has been popular since 1996. Whether you own a first-generation R170 (1996-2004) with the supercharged 230 Kompressor or naturally aspirated 320 V6, knowing common problems helps you stay ahead of costly repairs.

1. Convertible Top (Vario-Roof) Issues

The SLK's signature folding hardtop is brilliant when working, but problematic when it fails.

Symptoms

• Top stops mid-cycle or won't move at all
• Hydraulic pump runs continuously
• Warning message: "Visit Workshop"
• Leaking hydraulic fluid in trunk

Causes and Solutions

• Hydraulic Cylinder Failure: The five cylinders that operate the roof develop leaks. Rebuild kits available for $50-100 vs $2,000+ for new cylinders.
• Pump Failure: Located behind passenger seat. Replacement: $400-800.
• Microswitch Issues: Small switches detect roof position. Can often be cleaned.

2. Engine Misfires and Coil Pack Failures

Both the M111 (230 Kompressor) and M112 (320 V6) engines suffer from ignition coil failures.

Symptoms

• Rough idle, especially cold
• Loss of power under acceleration
• Check engine light with misfire codes (P0300-P0306)
• Fuel smell from unburned fuel

Solution

Replace all coils and spark plugs together. Use only Bosch or OEM Mercedes coils. Aftermarket coils often fail quickly. SLK 320 has 12 spark plugs (two per cylinder) and 6 coils. Budget $400-600 for quality parts.

3. Crank Position Sensor (CPS) Failure

A notorious issue across many Mercedes models of this era.

Symptoms

• Engine cuts out while driving, restarts after cooling
• Extended cranking before starting
• Random stalling at idle
• No start when hot

Solution

Replace crank position sensor. Located on the driver's side of engine block. Part costs $50-80, DIY-friendly repair taking 30 minutes.

4. K40 Relay Module (Early R170)

The infamous K40 relay module powers multiple critical systems.

Symptoms

• No start condition
• Cooling fans run constantly
• Fuel pump not priming

Solution

Located behind the battery in engine compartment. Can be resoldered by a competent electronics tech for $50, or replaced for $200-400.

5. Automatic Transmission Issues

The 5-speed automatic (722.6) used in SLK models has some weak points.

Symptoms

• Harsh shifting, especially 2-3 upshift
• Delayed engagement when selecting Drive/Reverse
• Limp mode (stuck in 2nd gear)

Common Causes

• Conductor Plate: Speed sensors fail, $200 part
• Low Fluid: Benefit from fluid changes every 40k miles
• Valve Body: Wear causes harsh shifting

Buying a Used SLK: Inspection Checklist

• Cycle the convertible top multiple times, hot and cold
• Check for misfires under hard acceleration
• Verify transmission shifts smoothly through all gears
• Check for rust, especially R170 models
• Scan for stored codes even if CEL is off

Last Updated: 2026-03-15