You make a change. You write the file. The car starts, runs for 30 seconds, then throws a fault code and goes into limp mode.
The ECU is performing a plausibility check.
What a plausibility check is
The ECU constantly compares sensor readings and calculated values against expected ranges. It’s not just “is this value within min/max” — it’s “does this value make sense given everything else I know?”
The three types of plausibility faults in calibration context
- Range check — a value fell outside its defined min/max
- Coherence check — two signals disagree beyond tolerance
- Dynamic check — the rate of change is implausible
How to work with this
Before changing any map, understand what other maps or sensors the ECU uses to validate that signal. DTC masking is a partial solution — but masking a fault without understanding the plausibility check you’re bypassing is how you create a calibration that passes the bench test and fails in the field.
We cover DTC behavior and coherence checking in detail in Diesel Practice and Gasoline Practice.