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?
Modern ECUs are connected to a large number of sensors in a standard passenger vehicle. The total amount is generally around 70, although this can increase with more complex and luxury models. Of these, up to 30 sensors monitor the engine, providing a constant stream of data to the ECU. This ensures that all the parameters within its calibration are within correct ranges.
When an aberration is detected in this data, the ECU will store the fault in its memory as a DTC. The possible number of DTCs programmed into an ECU can be well over 1000. If the check engine light illuminates on the dashboard, the driver will need to discover what has gone wrong.
A diagnostic tool plugged into the car’s OBD port can read out the stored DTCs. The driver then addresses the issues issues that have been highlighted.
This could be something really simple. For instance the P0457 code, which relates to the fuel evaporation sensor, probably means that you only need to replace the filler cap. But car ownership is rarely as straightforward and cheap as this, so codes which relate to far bigger problems are often an unavoidable fact of motoring life.
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 if the mechanical work that produced the DTCs in the first place has been carried out, and the engine is now running properly again; 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 problems initially will just trigger the codes once more, and the check engine light will remain present on the dashboard.
Deleting a DTC
Actual removal of a DTC can’t be done using a standard diagnostic tool: it can only be managed by reading out the engine calibration from the ECU and deliberately targeting 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.
These systems are often bypassed because fixing them is very expensive. There is an argument that vehicle manufacturers have incorporated them into their cars as cheaply as possible and that they’re simply not fit for purpose. Increasingly stringent environmental regulations, have forces manufacturers to find ways of bringing emissions down to acceptable levels. The common perception is that they have maintained profits whilst leaving the owners to foot the bill for systems that fail too easily, and too often.
The Worst Offenders
Adblue comes in for most criticism here. When it goes wrong it can cost thousands to remedy, often requiring a new tank, pump, and lines. It is perhaps unsurprising that car owners faced with this opt to just bypass the whole thing.
For this deletion to work, the associated trouble codes are then 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 standard 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. Without using the ECU Check service there is nothing he can do about it.
