DTCs - Clearing or Deleting?
There is a world of difference between triggering, clearing, and deleting a Diagnostic Trouble Code from an engine ECU
What is a Diagnostic Trouble Code?
A modern passenger vehicle uses around 70 sensors that continuously send data to the Engine Control Unit, although more complex and luxury models will use more. Of these, up to 30 sensors monitor the engine. The ECU compares this live data with its calibration to make sure every operating parameter stays within its specified range.
When the ECU detects a fault or an abnormal sensor reading, it stores a DTC in its memory. Manufacturers often program more than 1,000 DTCs into a single ECU. If the check engine light illuminates on the dashboard, the driver will need to discover what has gone wrong.
A diagnostic tool must be connected to the vehicle’s OBD port to read the stored DTCs, and identify the underlying fault.
This could be something really simple. For example, the P0457 fault code often points to a loose or faulty fuel filler cap, making it one of the quickest and least expensive problems to fix.
But car ownership is rarely as straightforward and cheap as this. Codes which relate to far more expensive problems are often an unavoidable fact of motoring life. Click here for a detailed example of the various types of DTCs.
Clearing a DTC
The technician who plugs into the car to read out the stored DTCs can “clear” them – effectively resetting the log that the ECU has created. This is fine assuming the mechanical work that produced the DTCs in the first place has been carried out, and the engine is now running properly. The dashboard will be free of warning lights and the vehicle can continue to be used.
But simply clearing the codes and hoping the problem resolves itself is an exercise in futility. The sensors that detected the faults initially will just trigger the codes again, and the check engine light will remain present on the dashboard.
Why Delete a DTC?
Increasingly strict emissions regulations have forced manufacturers to introduce highly complex control systems. These don’t age well, and they commonly fail.
These systems are, therefore, often bypassed because fixing them is very expensive.
As a result, many owners investigate ECU software modifications and DTC removal as an alternative to replacing costly emissions control components.
You can’t remove a DTC permanently with a standard diagnostic tool. Rather, the engine calibration must be obtained from the ECU, to target the codes that are being triggered. In diesel vehicles the exhaust’s DPF, Adblue system, and EGR valve are commonly removed. In petrol cars it is often the O2 sensor.
The Worst Offenders
Adblue is especially problematic. When it goes wrong it often requires a new tank, pump, and lines, with eye-watering bill to match. Faced with this, it is perhaps unsurprising that car owners choose to bypass the whole thing.
For this deletion to work, the associated trouble codes are also disabled within the ECU calibration. This stops the codes from being triggered at all, so the check engine light doesn’t come on. The car appears to run correctly.
Discovering whether or not this has been done to an engine ECU is impossible with garage diagnostic tools. The mechanic in our photo has spent a considerable amount of time trying to diagnose a problem on this car, but the DTCs he would expect to see on the tool are not present. As a result there is nothing he can do about it. However, when he uses ECU Check, he can get the information he needs.
