DTC Off Explained: Which Fault Codes to Disable on EDC16, EDC17 and SID ECUs

A customer’s Vectra C comes in with the EGR blanked by the previous owner, a P0401 on the tester and a lamp that comes back every second drive cycle. The mechanical job is done; the ECU simply doesn’t know it. That’s the situation DTC off is meant for, and it’s also where most bad files come from, because it’s tempting to switch off everything the tester shows and call it fixed.

What DTC off actually changes in the file

On Bosch EDC16, EDC17 and MED17 every fault is an entry in a diagnostic table (Bosch calls the entries DFCs). Each entry carries a class, a debounce time, a healing counter and flags for lamp, inhibit and limp reaction. Disabling a code means finding the entry, usually by its P-code or the internal name, and setting the class or the enable byte so the check never trips. On a well made file the environment data and the reaction (torque reduction, regen inhibit) are removed with it. On a lazy file only the lamp bit is touched, so the ECU still limps, it just doesn’t tell you.

Siemens/Continental SID and Simos units keep a similar table but the layout differs between SID201, SID803A and SID807EVO, so the same offset never carries across. Delphi DCM3.x and Marelli MJD hold the codes in a smaller, denser structure; on Marelli 6F3/8F3 a single wrong byte kills a whole group of checks. Denso ECUs on Toyota, Mitsubishi and Nissan are the hardest: the diagnostic is spread through the code and often needs a patch, not a map edit.

Codes that make sense to disable

• After hardware removal: EGR valve gone or blanked (P0401, P0403, P0404), DPF removed (P2002, P2452 to P2455, P244A), swirl flaps out (P2015, P2004), catalyst gone (P0420, P0430), MAF deleted (P0100 to P0103).
• Components with no economic fix left on an old car: a failed intake manifold flap motor on a VAG 2.0 TDI, a seized exhaust flap on a Mercedes V6, once the part is removed or fixed open.

The rule: the code should correspond to something that physically isn’t there any more, and the rest of the strategy should already be adjusted for it. Turning off P2002 on a car that still has a full DPF is not a fix, it’s a way to melt one.

Codes that should never be touched

• Misfire (P0300 to P0312). The misfire monitor protects the catalyst and the pistons. If it trips, something is wrong with a coil, an injector or compression.
• Knock sensor (P0325 to P0334). Without knock control the ECU cannot pull timing on bad fuel. On a MED17 turbo engine that is the fastest way to a holed piston.
• Rail pressure (P0087, P0088, P0191). A rail pressure code means the pump, the regulator or the sensor is failing. Disabled, the injectors run on a pressure the ECU only guesses at.
• Throttle and pedal sensor plausibility (P2135, P2138). The double-sensor check is the only safety net between a pedal fault and unintended acceleration.

By ECU family, from the bench

EDC16 (EDC16U1, EDC16C9, EDC16C39): the table is well documented, and codes disable cleanly. Watch the DFC for the intake flap on Opel Z19DT units such as OPEL VECTRA C EDC16C9 0281011616 55189631UK 1037372739 DPF EGR OFF; it sits next to the EGR entries and is easy to hit by mistake.

EDC17 (EDC17C46, EDC17CP14, EDC17C64): more entries, several per component, and the inhibit matrix that links a DTC to a limp reaction is separate from the table. A file like SKODA OCTAVIA EDC17C46 03L997018 9977 1037560941 has hundreds of DFC entries; a DPF off on this unit needs about twenty of them handled together.

MED17 and MEVD17 petrol: cat efficiency and rear lambda checks are fine to disable for a decat, misfire and knock never. On VW SCIROCCO MED17.5.5 03C997027 1249 1037516937 the rear lambda heater and ageing checks are separate entries; leaving one behind gives a P0141 a week later.

Does DTC off survive a dealer flash?

No. A software update or a campaign reflash overwrites the calibration and every disabled entry returns. Keep the modified file and the original read.

Will it pass inspection?

The OBD check at MOT/TÜV reads stored codes and readiness. A clean DTC off shows no codes, but if the underlying removal is an emission component (DPF, EGR, cat, SCR) the vehicle is still illegal on EU roads and a physical or smoke check can still catch it. We say this plainly on every emission-related file.

If you need codes removed, send us the full read (KESS3, KTAG, Autotuner, FoxFlash or a bench read with New Trasdata), the exact P-codes from the tester and a note on what hardware is gone. We build on your original, not on a generic file. Use the request form, or browse the EGR off and DPF off categories for solutions that already include the matching DTC handling.