A customer complains of poor mileage, a slight hesitation, a vibration at idle or a starting delay. You scan the vehicle and the screen says no fault codes found. The customer looks at you as if you have not done your job, and you are left explaining that "the scanner found nothing".
That answer is technically true and diagnostically useless. A clean code list does not mean a healthy vehicle — it means the ECU has not yet met its conditions for recording a fault. Most scanners used in Indian workshops display only one category of code, and it happens to be the smallest one.
1 The Five States of a DTC and What Each One Hides
| State | Meaning | What it tells you |
|---|---|---|
| Active / current | The fault is present at this moment. | Easiest to confirm — you can watch it live. |
| Pending | Detected once, not yet confirmed on a second drive cycle. | The earliest warning available. This is the code worth acting on before the MIL appears. |
| Confirmed / stored | Fault met the conditions and the MIL is switched on. | What most basic tools show, and all they show. |
| Permanent | An emissions-related fault that cannot be erased by any scanner. | Only the ECU clears it, after the monitor passes. Proof of a serious past fault. |
| History | Stored but not currently failing. | Tells you what has failed before, which is useful context for an intermittent complaint. |
2 Five Reasons a Real Fault Sets No Code
None of these are exotic. Taken together they explain most of the "scanner found nothing" vehicles standing in Indian workshops right now.
3 Readiness Monitors: The Evidence You Are Missing
Readiness monitors are the ECU's report card on itself. For each monitored system it reports ready (the test has run) or not ready (the test has not yet run since the last memory clear).
4 Mode 06: The ECU's Own Test Results
On vehicles and tools that support it, Mode 06 returns the raw results of the ECU's internal self-tests rather than a pass or fail verdict. Instead of waiting for a code to be set, you can see how close a system is to failing.
5 Building a Diagnosis From Live Data Alone
When there is no code, your evidence has to come from comparing what the vehicle is doing against what it should be doing. Write the numbers down — an unwritten reading is an opinion.
| Live parameter | What a healthy vehicle should show |
|---|---|
| Throttle position | A low, stable reading at closed throttle that rises smoothly and proportionally as the throttle opens — no jumps, no flat spots in the sweep. |
| Coolant temperature | A steady climb to normal operating temperature within a reasonable warm-up period, then stable. Slow warm-up suggests a stuck thermostat. |
| Oxygen sensor | Regular switching once warm. A sensor that holds a steady value, or switches slowly, damages economy long before it sets a code. |
| Short and long term fuel trim | Close to zero with small corrections. Values drifting consistently in one direction are telling you a fault is developing. |
| Battery voltage | Stable and above roughly 13.8V running. A low reading explains a whole family of "electrical" complaints that set no code at all. |
| Manifold pressure / airflow | Consistent with load, RPM and throttle position. An inconsistent relationship points to a leak or a drifting sensor. |
6 A Paid Service You Can Sell: The Pre-Purchase Scan
This same knowledge is a service Indian workshops can charge for, honestly and easily: scanning a used vehicle before someone buys it.
Be equally clear about the limits. A scan cannot reveal frame damage, a worn clutch, or a failing compression ring — say so in the report. Honest scope is what makes the service trustworthy enough to repeat.
7 Verifying the Repair and Using VahanScan
Related guides: MIL behaviour and reset procedure, and the sensor voltage specification guide for the physical tests that confirm what live data suggests.
8 Frequently Asked Questions
Why can a vehicle have a real problem but no fault code?
Because the ECU only records a fault when its monitoring conditions are met. Mechanical faults, sensors that have drifted but remain inside their permitted range, and intermittent faults that have not crossed the detection threshold all produce symptoms without setting a code.
What is the difference between a pending code and a stored code?
A pending code has been detected once but not yet confirmed, so the warning light is usually still off. A stored or confirmed code means the fault met its conditions and the MIL has been switched on. Pending codes are the earlier warning and are the ones worth acting on.
Why can a permanent code not be cleared?
Permanent codes are used for emissions-related faults and remain in the ECU until the vehicle completes the relevant monitoring and the fault is proven repaired. No scan tool can erase them, which is exactly why they are so useful when inspecting a used vehicle.
What does a readiness monitor showing "not ready" mean?
It means that particular self-test has not completed since the memory was last cleared. The absence of codes on such a vehicle is not proof of health — the tests simply have not run yet, which is why the vehicle needs to be driven through a full cycle before judging it.