Getting to the cause of underpowered Renault Megane II

Willie Barry of AGE explains how he got to the root of the problem on an underpowered 2005 Renault Megane II 1.9Dci.

Vehicle : 2005 Renault Megane II 1.9Dci

Problem : Low on power under 2500 rpm

Fault code : P0571 – Brake Pedal Switch Signal Improbable

And you guessed it – No management light on the dash.

First tested the brakes, no problem, then tested the brake lights, all fine, so why the code ?

Decided to test drive the vehicle and found it to be very low on power at low speed but felt sort of ok over 3000 RPM.

Started checking and testing all the mechanical engine components that can affect the low power issue, including induction, exhaust outlet and fuelling side.

Some Four weeks later, still nothing wrong, all components work to spec, no engine electronic fault codes, still no power.

The next day I needed to get a vehicle out in front of the Renault, so I started all the cars in that row in the workshop. When I got into the vehicle behind the idling Renault I noticed the centre brake light in the rear window was on. This vehicle has an auto lights on “only when the engine is running” function and with the vehicle idling in the workshop the lights were on.

But why was the centre brake light also on?

Switched off and popped under the dashboard to remove the 4pin brake light switch to test it – all ok.

Refitted the switch on the vehicle, pressed the brake pedal, all lights are working.
Fired it up again, lights on as well as the brake light, pulled the vehicle out to get to the one in front and all of a sudden the brake light goes off with the engine running outside in the sun, pressed the pedal, brake lights worked fine.

Test drove the vehicle again, still no power, back in the workshop, auto headlights came on and the pesky brake light was on again.

Confused even more I sent the car to the sparky to sort out the brake light wiring while I’m planning on what to do about the flippin low power issue.

Sparky rang me inside an hour and tells me the brake light problem is sorted. Went to pick up the vehicle and he hands me the old brake light bulb, saying there should be double contact bulbes fitted, in one light a single contact bulb was fitted, shorting the two pins. So when the auto lights activated, the single contact bulb took the rear driving light power and send it back into the brake light switch, logging the fault code “Brake Pedal Switch Signal Improbable”.

When the fault code is activated the ECU reduces the engine output.

Repair : Fitted 2 new double contact bulbs and the car ran fine again.