Faulty economy mode switch, for example internal short or a stuck button; Signal wire shorted to ground through chafed or damaged wiring; Corroded, loose or water-damaged connector at the switch or control unit; Open circuit or high resistance in the switch supply or reference feed giving a low reading; Moisture or debris contamination in the switch or its surrounding console area, for example after a spilled drink; Fault inside the control unit that reads the switch.

P05C9

Economy Mode Switch Circuit Low.

Generic
Powertrain
Generic code
Low severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
5: AreaVehicle speed, idle control and auxiliary inputs
C9: FaultThe specific fault in that area

What it means

P05C9 means the control unit has detected a low voltage signal in the economy mode switch circuit, which is the input the driver uses to select a fuel-saving drive mode. The module saw a voltage lower than the expected range for the switch's current position, often suggesting a short to ground or an open or faulty switch. Exact switch design, signal voltage and which module monitors it vary by model, so check the manufacturer's data for the vehicle.

Symptoms

Economy mode cannot be selected or does not stay selected
Economy mode indicator light not working or behaving oddly
Warning light on, such as the engine management light, depending on the vehicle
Changed throttle response or gearbox shift behaviour compared with normal
Drive mode selection works intermittently or not at all

Possible causes

Faulty economy mode switch, for example internal short or a stuck buttonlikely
Signal wire shorted to ground through chafed or damaged wiringlikely
Corroded, loose or water-damaged connector at the switch or control unitpossible
Open circuit or high resistance in the switch supply or reference feed giving a low readingpossible
Moisture or debris contamination in the switch or its surrounding console area, for example after a spilled drinkpossible
Fault inside the control unit that reads the switchrare

How to diagnose

Read all stored and pending codes and note the freeze frame data, then check for related drive mode or switch circuit codes
Check whether the economy mode switch and its indicator respond, and note whether the fault is constant or intermittent
Visually inspect the switch, its surroundings and the wiring for damage, liquid ingress, chafing or previous repairs
Disconnect and inspect the switch and control unit connectors for corrosion, bent pins and poor terminal tension
Using the manufacturer's wiring diagram, measure the supply or reference voltage and the signal voltage at the switch with it pressed and released
Check the signal wire for a short to ground and the circuit for continuity and resistance with the battery disconnected, following the manufacturer's procedure
Test the switch itself for correct resistance or continuity in each position against the specification
Clear the code and road test to confirm it does not return, after checking for any manufacturer software updates relevant to the model

How to fix

Replace the economy mode switch if it tests faulty or is contaminated
Repair or replace damaged wiring that is shorted to ground, using proper soldered or crimped repairs
Clean or replace corroded or damaged connectors and terminals
Restore the correct supply or reference voltage by repairing the open circuit or high resistance in the feed
Replace or reprogram the control unit only after the wiring and switch are proven good and the manufacturer confirms it is necessary

Can I keep driving?

Low severity

The car is normally safe to drive as the fault mainly affects the economy mode function, but have it checked soon so the fault does not hide other problems. If you notice unusual throttle or gearbox behaviour, reduce your driving and book it in sooner.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.