Recurring fault codes such as P0606 and P1337 occur regularly on the Citroën C4. They often point to deeper problems with the ECU (engine control unit) or to adaptation values (learnt settings) that have not been reset after repairs. These codes can be triggered by high voltage, incorrect signals to the ECU or incomplete reset procedures. With an OBD2 scanner (diagnostic tool for reading fault codes) you can read these faults yourself, understand the cause and in many cases solve the problem without going straight to a garage. Find out how to tackle these frustrating engine faults.
What fault code P0606 means and how to fix it
Fault code P0606 stands for an internal processor error in the ECU. This code indicates that the engine ECU no longer trusts itself and signals that internal checks have failed. This can be caused by:
- Sudden voltage spikes when starting or charging the battery
- Corrosion or moisture in the ECU housing
- Defective internal components after years of use
- Software incompatibility after updates or flashing
The direct consequences are often an illuminated engine warning light, starting problems or a car that goes into limp mode. The result is that the car activates limp mode and engine power is reduced. The solution starts with reading the code via the OBD port. First check the battery and earth connections for corrosion. Disconnect the battery for 15 minutes to fully reset the ECU. Then start the engine and drive at least 20 kilometres at varying speeds. This gives the ECU the chance to recalibrate.
Does the code come back? Then there may be a hardware defect in the ECU. Some owners of the Citroën C4 report that a professional ECU reset (engine computer) or reprogramming at an authorised dealer solves the problem. Costs range between €150 and €400, depending on the year of manufacture and the garage.
Recognising fault code P1337 in injector faults
Code P1337 indicates a problem with the injectors (fuel injectors). It also points to faults in the fuel control system. This error occurs regularly on C4 models with diesel engines, especially after 150,000 kilometres. The causes are:
- Contaminated or leaking injectors due to poor fuel quality
- High electrical voltage disrupting injector control
- Adaptation values not reset after injector replacement
- Defective wiring connectors to the injectors
The main symptom is an irregular engine idle, especially on cold starts. The engine may stumble or struggle to idle smoothly. After replacing injectors you should always reset the adaptation values (learnt settings). Use a professional diagnostic tool or brand-specific scanner for this. If you do not, the ECU will continue to work with old values and the code may return.
Also check the electrical wiring to the injectors for damage. Replace damaged connectors immediately to prevent further faults. Clean the injectors with a specialised cleaner if they are not yet completely defective.
Reading OBD codes yourself when you have engine faults
You read fault codes with an OBD2 scanner. You plug this into the OBD port, which on the C4 is located under the dashboard on the driver’s side. Follow these steps:
- Switch off the ignition and connect the scanner to the OBD port
- Switch on the ignition without starting the engine
- Select the option ‘Read Codes’ or ‘Uitlezen’ on the scanner
- Write down all displayed codes and descriptions
- View freeze-frame data to see under which conditions the fault occurred
A budget-friendly OBD2 scanner costs between €30 and €50 and reads most engine management codes. For deeper diagnosis of systems such as ABS (anti-lock braking system), airbags or the transmission you need a brand-specific tool. One example is the iCarsoft i970 for PSA brands. This costs around €150 but gives access to all modules of the car.
Note: do not erase codes without first fixing the cause. Otherwise the fault will return within a few kilometres and you will lose valuable diagnostic information.
Resetting the ECU after repairs on the Citroën C4
After replacing parts such as sensors, injectors or the turbo you must reset the ECU. This ensures that the computer relearns with the new components. There are two methods:
Method 1: disconnecting the battery
- Write down radio and infotainment codes so you can re-enter them
- Disconnect the negative terminal of the battery
- Wait 20 to 30 minutes so that all residual voltage dissipates
- Reconnect the battery and start the engine
- Drive at least 30 kilometres at varying speeds and loads
Method 2: using a scanner
With a professional scanner you can selectively erase adaptation values. This way you do not reset all settings at once. It is more precise and prevents settings such as window calibration from being lost. Select ‘ECU Reset’ or ‘Clear Adaptations’ in the menu and follow the on-screen instructions.
Owners report that a thorough reset is especially necessary after replacing the MAF sensor or lambda sensors. Otherwise the engine may continue to hesitate or show higher fuel consumption.
When recurring codes require professional help
Some fault codes keep coming back despite resets (erasing fault codes and settings). They also return after replacing parts. This points to underlying problems that require deeper diagnosis. Go to a garage when:
- The same code reappears within 50 kilometres after being erased
- Several unrelated codes are active at the same time
- The car goes into limp mode and power is limited
- The codes relate to safety systems such as ABS or airbags
Authorised Citroën garages have factory diagnostic equipment. This provides access to all software layers and hidden codes. Costs for professional reading are between €30 and €60. For complex faults a complete ECU diagnosis including software update may be necessary, costing €150 to €300.
If you are unsure about electrical faults or recurring P0606 codes, professional help is the safest choice. An incorrect diagnosis can lead to unnecessary repairs and higher costs.
Preventive measures against fault codes
Many fault codes on the C4 can be prevented through regular maintenance. This avoids problems before they arise. Replace engine oil according to the manufacturer’s recommendations and always use PSA-approved specifications. Check the battery voltage regularly, especially on cars older than five years. A weak battery causes voltage spikes that can damage the ECU.
Check connectors and wiring in the engine bay for moisture and corrosion. Spray vulnerable connections with contact spray to prevent oxidation. Replace air and fuel filters on time to protect sensors from contamination.
On diesel models, adding injector cleaner every 10,000 kilometres helps prevent blockages. Avoid extended idling. This encourages soot build-up in the EGR valve (exhaust gas recirculation valve) and the DPF (diesel particulate filter).
Discover more about Citroën problems and maintenance
On the Carnews website you will find extensive information about common faults on Citroën models, maintenance schedules and practical repair tips. Whether you want to work on the car yourself or prepare for a workshop visit, there is much more to discover. Check out the other articles and expand your knowledge of automotive technology and troubleshooting.
Frequently asked questions
You need at least an OBD2 diagnostic tool (scanner) that you plug into the OBD2 port under the dashboard of your Citroën C4. For basic engine management codes a universal OBD2 scanner is sufficient (for example a simple handheld model or a Bluetooth dongle with app). If you also want to read ABS, airbag and BSI/comfort systems, you need a brand-specific Citroën/PSA diagnostic tester (such as an iCarsoft for Citroën/Peugeot). If necessary, a professional diagnostic computer at the garage can be used for deeper analysis and software actions.
You can read the fault codes on your Citroën C4 via the OBD2 port under the dashboard with an OBD2 scanner; universal scanners mainly read engine management, brand-specific ones (such as iCarsoft for PSA) also read ABS, airbags and the BSI. Write down the code (e.g. P0171 or P0420) and look up the meaning in an OBD code list or app to see which sensor or system is reporting a problem. First fix the cause (e.g. spark plugs, lambda sensor, cleaning the EGR) and only then erase the fault codes with the scanner, otherwise they will soon return. If fault codes remain or they relate to brakes/airbags, have them read by a garage or PSA specialist.
Yes, there are a number of fault codes that occur regularly on the Citroën C4, such as P0171 (fuel mixture deviation), P0300 (random misfire), P0420 (insufficient catalytic converter efficiency) and P0401 (insufficient EGR flow). The causes are often contaminated or defective sensors (lambda/MAF), worn spark plugs or ignition coils, a blocked EGR valve or a worn catalytic converter. Fuel pressure problems (P0001) also occur due to a defective pump, sensor or clogged filter. In addition, the C4 is regularly affected by electronic issues, such as moisture in connectors or faults in the BSI module.
It is only sensible and safe to reset a fault code yourself if you have identified the cause of the fault and have fixed it (or had it fixed), for example after replacing spark plugs or a sensor. Use a suitable OBD2 or brand-specific PSA scanner and check that the car drives normally and shows no strange noises, smoke or loss of power. Do not reset fault codes for engine, brake, airbag or ABS faults if you are not sure what is going on; in that case go to a garage. Also bear in mind that a fault code that reappears immediately or very quickly indicates a problem that is still present and must be solved first.
First check the fault code with an OBD2 scanner and note the exact code and description. Then look for the cause (for example sensor, ignition coil, EGR, catalytic converter) and fix the underlying problem before erasing the code. Only then erase the fault code via the scanner and take a test drive of at least 15 minutes to check that the warning light stays off. If the fault code returns again, have a professional diagnosis carried out (for example for ABS/airbag codes or recurring engine faults).