Codes and warning lights
I have several trouble codes at once: which one should I fix first?
Start with the ones that can cause the others: battery voltage or power supply, lost communication between modules (U codes) and sensors shared by several systems. Then look at which one was stored first and leave for last the ones that are usually a consequence, such as catalytic converter codes after misfires.
Reviewed by Daniel Morales, founder of Reprorace SL, on
Why do so many codes show up together?
A weak battery, a jump start or a loose terminal can leave voltage and communication codes in many modules at once. The same happens with a fault in the CAN network.
In that case, once the power supply is fixed and the codes cleared, most of them do not come back.
Which ones are urgent?
Codes for brakes, airbags, steering, high voltage in hybrids, oil pressure and temperature, and misfires with a flashing check engine light. Those come before anything else.
An emissions code with the car running fine can wait a few days, but not weeks.
How do I know which one came first?
Many scanners show the code that triggered the freeze frame data. The environment data of each code, such as mileage or voltage, helps you piece together what happened.
Write down every code and its status before clearing anything: clearing erases that trail.
General guidance only: every manufacturer and model has its own details. Always follow your vehicle's manual and, if in doubt, ask a workshop.
Related
Related trouble codes
- U0100Lost communication with ECM/PCM A
- P0562System voltage: too low
- P0300Random or multiple cylinder misfire detected
- P0420Catalyst system (bank 1): efficiency below threshold
Related symptoms
- Battery warning light on: what it means and what to do
- Engine warning light on and steady: what it means and what to do
Learn more
See what your car is saying
The ECO-LITE reads the modules that answer through the port and shows you the codes sorted by system, and your shop can see the data of each one with your permission.
Presale price: €129 · US$129
Frequently asked questions
Do many codes mean a serious fault?
Not necessarily: they often all come from a weak battery or a bad connection.
Should I clear them all and see which ones return?
Only after writing them all down with their data; it is a useful test, not a repair.
What if there are U codes in several modules?
Check the battery, the grounds and the connectors first; if they persist, have a shop test the network.