P0790

Code

MERCEDES-BENZ P — Powertrain

Normal/Performance Switch Circuit Malfunction

Brand: MERCEDES-BENZ

Type: P — Powertrain

Views: UK: 21 EN: 36 RU: 31

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Page language: EN

Causes

Faulty Normal/Performance mode switch (on stalk, console or transmission)
Open, shorted or corroded wiring or connectors in the switch circuit
Poor ground or reference voltage to the switch
Blown fuse or relay feeding the switch/TCM
Intermittent connection (water ingress, damaged pins)
Faulty TCM/ECM or internal driver failure

Symptoms

MIL/CHECK ENGINE lamp illuminated
Cannot select or stay in Performance/Normal mode
Transmission shift strategy abnormal or stuck in limp mode
Reduced engine or shift performance
Intermittent or permanent loss of mode indicator on dash

What to check

Read and record all stored codes and freeze-frame data with a proper OBD-II scan tool
Confirm whether the fault is active or historical and whether it is intermittent
Visually inspect switch, connector and wiring for damage, corrosion or water intrusion
Check relevant fuses and relays
Backprobe switch connector and observe signal while operating the mode selector
Measure reference voltage and ground at the switch connector

Signal parameters

Mode switch is typically a two-state digital signal — expected states commonly near 0 V and battery/logic voltage (often 5 V) or open/ground. Confirm exact voltages in OEM wiring diagram.
Reference supply (Vref) usually 5 V or ignition-switched battery; measure present and stable with ignition on
Switch continuity: near-zero ohms between terminals when closed; open when actuated (or as specified by OEM)
No more than a few ohms of harness resistance; no short to ground or battery

Diagnostic algorithm

Connect a scanner and record P0790 and any related codes; note freeze-frame and live-data values for mode switch state.
Attempt to reproduce the fault by cycling Normal/Performance modes while watching the live data and stored PID for the switch.
Visually inspect the switch assembly, harness and connector for damage, corrosion or moisture; repair any obvious issues.
With ignition ON (engine off), backprobe the switch connector and verify reference voltage and ground are present per OEM spec.
Operate the switch and observe the signal line; confirm it toggles cleanly between expected states without intermittent readings.
Perform continuity and resistance checks from the switch pins to the TCM connector; repair any open or high-resistance runs.
If a short to battery or ground is suspected, isolate sections of harness and retest until location is found.
If wiring and switch test good, consider swapping with a known-good switch (if serviceable) or consult OEM procedure for switching module/TCM tests.
Check for available TCM/ECM software updates and reflash if directed by OEM service information.
After repair, clear codes and perform a test drive to confirm the fault does not return.

Likely causes

Corroded or loose connector at the mode switch
Broken or pinched wire in switch harness
Failed switch contact (worn or stuck)
Loss of reference voltage/ground to the switch

Fault status

⚠️ Status

Control module detected abnormal or out-of-range signal on the Normal/Performance switch circuit. The condition may be due to a faulty switch, wiring problem, poor reference/ground or module fault. Diagnostic testing of switch, harness and control module required.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours

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