Diagnostic Trouble Code P0504 stands for mismatch of signals from sensors 1 and 2 brake pedal or Brake Switch A/B Correlation. Appears when the ECU receives conflicting information about the current position of the brake pedal.
Most often, error P0504 is recorded in Lada (Kalina, Granta, Priora, Vesta, Largus), Datsun (On-DO, mi-DO), Renault (Logan, Duster), Toyota (Corolla, Land Cruiser Prado) and other models, in in which this information is transmitted to the controller from the four-pin brake light switch.
We will tell you why error P0504 occurs and how to find and eliminate the cause of discrepancies in brake sensor signals in this article.
Symptoms and Conditions for DTC P0504 Appearance
The formation of DTC P0504 is caused by the receipt of certain signals into the electronic control unit from the light limit switch , also known as the “frog”. In a number of cars, in particular Lada models with an electronic accelerator pedal, two pairs of contacts are combined in its body: a normally closed one for the brake pedal position and a normally open one for the brake light circuit. When the pedal is pressed, the first contact opens and the second contact closes.
Information about the state of both contacts is sent to the ECU. If their values do not correspond to each other, for example, both contacts are closed or open, or simultaneously with the information about the brake pedal being pressed, the accelerator pedal is pressed, error P0504 occurs .
Since this algorithm ensures safe operation of the vehicle, if the E-gas system malfunctions, if the signals from the brake and accelerator pedal position sensors diverge multiple times, the engine may go into emergency operation mode .
Therefore, together with error code P0504, certain symptoms may appear:
Decoding error P0504 in the diagnostic program
- The brake light lights up constantly or does not light up when the brake pedal is pressed.
- Cruise control does not activate.
- The dynamics deteriorate, the engine speed does not rise above 1250–1500 per minute and/or drops to idle during acceleration, the engine stalls or does not start.
- On cars with automatic transmission, problems with changing the position of the selector may occur.
- The Check Engine light on the dashboard lights up (usually when the error is detected multiple times).
The set of symptoms directly depends on the nature of the malfunction, the logic of the ECU and the design features of a particular model. In some cases, the malfunction may not reveal itself as anything other than an error code in the memory.
Grant brake pedal frog
Hello everyone! I would like to tell you a little about the M74 ECU and the common error “P0504 Incorrect signal from the brake pedal switches.” In connection with the transition to new toxicity standards, AVTOVAZ engineers and designers have developed a new line of M74 and M17.9.7 ECUs. The ECUs can be said to be normal, but have not yet been fully tested. After all, cars on these ECUs are very much “strangled” by toxicity standards and therefore, without reprog and installing firmware e2 or lower, you can’t drive the car in any way, but this action is not all available now, since the protocol, pads and the reprog algorithm itself have changed with the ECU, in Therefore, it is necessary to have an additional module for the programmer. Diagnostics of ECU data is already available, since all major companies have taken care of their customers and have released software updates.
Well, okay, I’ll start right away with the story of how to get rid of the error “P0504 Incorrect signal from the brake pedal switches.” Due to the fact that the ECUs were changed, AVTOVAZ began installing new sensors and IMs. There are also 2 new sensors on the brake pedal, just don’t be alarmed if you suddenly find only one. AVTOVAZ designers and engineers managed to combine 2 sensors in 1 housing, that is, only one sensor is installed on the brake pedal, but this is not so, inside the housing there is the brake pedal sensor itself and another sensor, which is popularly called “frog”. Error “P0504 Incorrect signal from the brake pedal switches” appears because the sensors are not adjusted properly from the factory. If suddenly a car with M74 or M17.9.7 came to you and the scanner produced such an error, then you can safely take the keys and adjust the sensor gap, since it is not adjusted. The sensor is adjusted in a simple way and is not difficult. There are two nuts on the sensor body. One secures the sensor to the mount from above, and the other from below. The nut that is on top regulates the sensor gap, so loosen the bottom nut a little, and then tighten the top one a little, thereby adjusting the sensor gap. In my case, the sensor was set from the factory so that the button was pressed only halfway, now I made it press almost completely. I say almost due to the fact that I left a small reserve of travel for the button, about 2 mm, so that when the brake pedal is suddenly released, the button does not get too clogged into the sensor. When you adjust the gap, then start the car, having first reset the error, and let it run for a while, since the error is not registered immediately, but after a certain period of 5-10 minutes. If the error is not registered, then you can safely take money from the client and say goodbye to him. Also, on some cars, a clutch pedal sensor is installed; it also needs adjustment, because if it is not adjusted, the car begins to stall, as it were, since the ECU thinks that the clutch pedal is depressed. I guess I'll finish here. If new information appears on the ECU data, I will post it on this forum, since it is the best.
Reasons for mismatch of brake sensor signals and their elimination
In most cases, error P0504 occurs due to a malfunction of the brake light limit switch or its incorrect adjustment (if required). However, failure is also possible if non-standard lamps are installed in the rear lights. The main reasons for mismatch of brake sensor signals and methods for eliminating them are discussed in the table.
| Causes and solutions to error P0504 | ||
| Faulty element | Reason for failure | How to fix |
| Stop light limit switch (“frog”) | Incorrect adjustment | Adjust the position so that the rod is completely recessed into the body when the pedal is released. |
| Spring is weakened or broken | Replace the spring or change the sensor itself | |
| Contacts are burnt or oxidized | Clean the contact plates with sandpaper or a file, or completely replace the end switch | |
| Brake lamps | Burnt out filament | Replace lamp |
| Installed non-standard lamps with lower resistance, for example, LED | Add a resistor to the circuit, replace with lamps with blende or standard ones | |
| Fuse | Brake light circuit fuse blown | Replace the fuse, if it blows again, check the circuit for short circuit |
| Circuit between limit switch, ECU, brake lights and battery | Broken wires, damaged insulation or contact terminals | Visually check the integrity of the wires and contacts, test the wiring with a tester or test lamp |
| ECU | Damage to contacts, failure of elements on the controller board | Replace contacts and faulty components |
After eliminating the cause of the malfunction, it is necessary to reset error code P0504 using a diagnostic scanner and a special application, otherwise the engine may continue to operate in emergency mode.
Diagnostic procedure when error P0504 appears
To carry out some diagnostic operations when error P0504 appears, a multimeter or test lamp . If you do not have an assistant to check the operation of the limit switch and the condition of the brake lamps, you can drive up to the wall so that the brake lights are visible in the mirrors.
- Check the brake lights by pressing and releasing the brake pedal several times. The lamps should light up when the brake pedal is depressed 2–3 cm and go out when released. Flashing or flickering indicates poor contact, while intermittent failure indicates incorrect adjustment or wear of the limit switch.
- If the lamps do not illuminate when the pedal is depressed, check that the brake light circuit fuse has not blown.
- Inspect the limit switch connector, make sure it is fixed and there is no external damage.
- Check the brake light limit switch and its adjustment. The verification procedure is discussed in detail in the next section.
- Check the power supply to the sensor using a multimeter or test lamp. One of the terminals must be supplied with 12 V power at all times. Use the vehicle's electrical diagram to determine the appropriate terminal. If there is no power, check the corresponding circuit in sequence.
- If there is power and the sensor is working, check the receipt of signals when the pedal is pressed and released to the corresponding contacts of the computer using a multimeter. Their presence indicates a controller malfunction.
If error P0504 appears after replacing brake light bulbs with LED ones, first of all try installing back the standard halogen ones. Most likely this is the problem.
End switch connection diagram, click to enlarge
Brake sensors for bicycles where replacing brake levers is not possible
As we already found out in the first section of the topic, brake sensors are built into the brake handles of an electric bicycle. They are activated when the trigger is pulled and send a signal to the control unit, which in turn stops the power supply to the motorized wheel.
However, an unpleasant situation arises here: when converting a traditional bicycle into an electric bike, replacing the handles is sometimes an impracticable technical maneuver. On a bicycle with a hefty price tag, the brake levers can be made in a single housing with the gear shifter. Unfortunately, replacing such products with modifications with sensors will not work. However, we are still resolving this issue.
For such bikes, sensors have been specially developed that are embedded in the brake cables. You will have to pay more money for such devices than for brake handles equipped with sensors. But sending a signal to the control unit is not their only function. These parts are also equipped with red LEDs, which are activated when braking, acting as a “stop”. Of course, such a glow is not very noticeable in the daytime, but in darkened conditions a single light manifests itself in all its glory.
Brake sensors mounted on the brake cables have a third wire. This is a little unusual, since standard handles equipped with brake sensors use a 2-wire system. So, when you select components for electrifying a bicycle using sensors in the brake cables, remember this, because you will need a control unit with the appropriate connector.
Checking, repairing and adjusting the brake light switch on cars of different brands
Inside the four-pin brake switch there are two pairs of contacts, the state of which depends on the position of the rod: when fully recessed and fully extended, one pair is always closed - the resistance is close to zero, the other is open - the resistance is several kOhms or more. If for some reason the contact group does not work properly, error P0504 occurs.
Checking the brake light switch
Checking the brake light switch comes down to testing the terminals of a pair of contacts in the normal position - with the rod extended and when it is retracted, simulating pressing the brake pedal. The normal state of the contacts of a working switch for some popular models is shown in the table below.
| Model | Contact status | |
| The rod is recessed | The rod is extended | |
| Toyota Rav4 | 3–4 closed 1–2 open | 3–4 open 1–2 closed |
| Lada (Priora, Kalina and Granta, 4x4) Datsun (on-DO, mi-DO) Gazelle Next, Business | 1–4 open 2–3 closed | 1–4 closed 2–3 open |
| Lada (Largus, Vesta, XRAY)Renault (Logan, Duster) | 3–4 open 1–2 closed | 3–4 closed 1–2 open |
If the test shows that the contacts close and open properly, error P0504 may be due to either incorrect adjustment of the brake light switch (not required on all models) or faulty wiring and other reasons listed above. If the multimeter readings do not match the parameters of a working switch, it must be replaced or repaired.
Repair of brake pedal limit switch
Diagnostics and repair of brake light switch: video
The two main reasons for the failure of the brake light limit switch are oxidized or burnt contacts and a broken or weakened spring. An indirect sign of the second problem is a small stroke of the rod compared to a good one or its free motion.
For repairs, the limit switch must be dismantled. To do this, first of all, disconnect the connector by releasing the latch. In some cars, for example, Lada Kalina and Granta, this requires unscrewing the fastening nut, in others, for example, Lada Largus and Vesta, it is enough to turn its body counterclockwise 90 degrees and remove it from the hole.
Next, you should disassemble the sensor housing, which is secured with latches, and then inspect the condition of the spring and contacts. Oxidized or burnt contacts can be cleaned with sandpaper and treated with a degreaser. The broken spring needs to be replaced with a new one of the same length (for the switch 1118-3720010 installed on Lada cars, a spring with a diameter and height of 6–8 mm is suitable), and you can try to stretch the weakened one, but this will only help for a while.
Repairing the brake pedal end switch is more of a temporary measure in case it was not possible to quickly find and purchase a replacement part. Usually, soon the restored switch begins to work incorrectly again. Therefore, if possible, it is better to install a new switch immediately.
Replacing and adjusting the limit switch: video
Purpose of brake sensors
Imagine this situation: you rode an electric bike to some public event, met with friends, slowed down and started communicating with them, but you forgot to turn off your electrified iron horse. Here, at some point, one of your friends approached your device, who had never dealt with such equipment before and, for fun, decided to twist the gas handle, completely unaware of the consequences of these manipulations. Naturally, the two-wheeled product will start moving.
You also did not expect such a whim of fate, so you can either fall yourself or crash into a citizen who is serenely positioned in your path.
A similar situation can happen to a rider if he is stopped at a crossing, waiting for the right moment when he can cross, and someone accidentally, or maybe the cyclist himself, touches the pedal of the electric bike, which triggers the PAS sensor. The device will move onto the roadway, probably together with the rider, and the more power the unit has, the more unpredictable the outcome of the unexpected maneuver will be.
Just so that such extreme misunderstandings do not arise, sensors are mounted on the brake levers that allow, even with a weak impact on the brake control, to turn off the power supply to the power unit. This saves battery power and also prevents unnecessary overloading of the bicycle’s electric motor during braking.
Another purpose of brake sensors is to enable recuperation (if provided) on electrified bicycles and motorcycles with direct drive engines.
Well, next, we will discuss the types of brake sensors.
