As of September 2026, a security light with a no-start is an immobiliser fault about half the time and something else the other half — and the half-hour of free diagnosis that separates the two is the difference between a $100 file, a $100 to $250 programming session, and a job that needed no ECU work at all. The test that splits them costs nothing and takes ten seconds: does the engine crank?
This post is the routing logic, written for the person standing next to the car rather than for a bench. It is deliberately sequenced so that the cheap checks come first, because a meaningful share of the immobiliser jobs we are asked to quote turn out to be a battery, a key battery, or a starter.
Step one: crank or no crank
A factory immobiliser system does not prevent cranking on most platforms. It prevents the engine running— by withholding fuel, spark, or the authorisation the engine controller needs before it will command either. The classic signature is an engine that spins normally, fires for half a second, and dies.
If it cranks and immediately stalls, the immobiliser branch is live and worth pursuing.
If it cranks and never fires at all, the immobiliser is one candidate among several; fuel and spark are just as likely and cost nothing to test first.
If it will not crank, the immobiliser is almost certainly not your problem on the majority of platforms. Check battery voltage under load, the neutral safety or clutch switch, and the starter circuit before anyone quotes you for module work. A handful of platforms do gate the starter relay through the body module, so this is not a universal law — but it is the right order to check in.
Step two: read what the light is actually telling you
Security and anti-theft telltales behave differently depending on state, and the behaviour is diagnostic in itself.
| Light behaviour | Usually means | First thing to check |
|---|---|---|
| Flashes slowly, key out | Normal armed state | Nothing — this is the system working |
| Goes out on key-on, car starts | Key authenticated normally | Look elsewhere for the fault |
| Stays on solid, engine stalls | Authentication failed or timed out | Second key; key battery; ring antenna |
| Rapid flashing on key-on | Transponder not seen at all | Antenna ring, key chip, key position |
| Never illuminates at all | Cluster, bulb or network fault | Network codes before immobiliser codes |
| Came on after module work | Configuration incomplete, not theft | What was replaced, and what was skipped |
Step three: the second key
Try the spare. It is free, it takes thirty seconds, and it splits the diagnosis cleanly in half.
If the spare starts the car, the fault is in the first key — a dead battery in a proximity fob, a cracked transponder, worn blade contacts. That is a key problem and a locksmith problem, and no ECU work is involved.
If neither key works, the fault is in the vehicle: the antenna ring around the ignition lock, the immobiliser control unit, the data stored in it, or the link between it and the engine controller. That is the branch where our work starts.
If you have no second key at all, you are in the all-keys-lost case, which has its own routing entirely — our all-keys-lost routing guide covers when that is a file job, when it is a session, and when the module has to come out.
Step four: pull the codes, and read them in the right order
Generic OBD scanners frequently show nothing at all on an immobiliser fault, because the relevant codes live in the body or immobiliser module rather than in the powertrain module. A scan that reads all modules is worth the extra minute.
Two body codes carry most of the diagnostic weight here. B2784is an engine immobiliser authentication failure — the system asked the key a question and did not get the right answer back. B2103 is an antenna coupling coil circuit failure, which means the ring that reads the transponder is not working, so the key never got asked at all. Those are different faults with different fixes, and a scan that stops at the first code misses the distinction.
Network codes change the picture again. If the immobiliser module and the engine controller cannot talk to each other, the engine controller will refuse to run for want of an authorisation it never received — and the light is a symptom of a communication fault rather than a theft-protection event. Our index of which codes blame the module and which mean wiring is the sorting guide for that.
“Nine times out of ten the person who calls me about a security light has not tried the other key. It is not that they are careless — it is that the light looks like a verdict, so nobody thinks to test it.” — Remote programming desk, ECU Flash Cartel
Step five: did anything change recently?
The single most useful question, and the one most often left out of the first message.
A module was replaced.Then this is not a theft event at all — it is an unfinished job. A replacement engine controller arrives either virgin or carrying another car's immobiliser data, and until it is married to this vehicle the security system is doing exactly what it was designed to do. Our post on why a swapped ECU will not start walks the whole failure tree for that case, and it is the one to read before buying anything else.
The battery was disconnected or went flat. Some platforms come back from a deep discharge with adaptations lost and a module in a confused state. Often a correct reconnection and a relearn clears it.
An attempted theft. A damaged ignition lock, a forced door, a broken column shroud. Here the immobiliser genuinely did its job, and the repair is mechanical first.
Somebody else already tried to fix it. A half-completed key programming attempt, a write that did not finish, a module ordered from an auction site and fitted without configuration. This is the case where telling us up front saves the most money, because the diagnosis is completely different from a car that simply stopped working one morning.
Three situations that look identical from the driver's seat
All three present as a car that turns over and will not run, with a lit security lamp, and they have almost nothing else in common.
The key is not being read. The transponder, the antenna ring around the lock, or the wiring between the ring and the immobiliser module. This is the cheapest branch and the one worth eliminating first, because the fix is frequently a part that costs less than an hour of diagnostic time.
The key is read but rejected. The immobiliser recognised a transponder and decided it is not one of its registered keys. That is a data problem rather than a hardware one, and it is what a key registration session exists to fix.
The key is accepted but the engine controller is not answering. Authentication succeeded and the controller that should act on it is silent. Now you are in network and power territory rather than security territory, and the codes will say so.
Distinguishing them is what the code read in the previous step is for. A car whose owner tries a single key, gets nothing, and immediately buys a used immobiliser module has skipped the only cheap branch of three.
What to have ready before you call anyone
Whether the next call is to us, to a locksmith or to a dealer, the same six facts determine how fast the conversation converges.
Year, make, model and engine, and the VIN. Several platforms changed immobiliser architecture partway through a model year, so the VIN settles what the model year alone cannot.
Crank or no crank. The first branch of the whole diagnosis, and the one people most often omit.
What the lamp does— steady, flashing, or off entirely — and at which point in the key cycle it changes.
How many keys exist, and whether any of them has ever worked. A car with one key is a different job from a car with three, and all-keys-lost is a different job again.
Every code, from every module. Not just the powertrain codes a basic reader shows.
What changed recently. Battery, alternator, module replacement, body repair, a locksmith visit, a jump start. The honest answer here shortens the diagnosis more than anything else on the list.
A caller who has those six ready gets routed to the right service on the first message. A caller who has none of them gets asked for them, which is the same conversation a day later.
Where each branch goes, and what it costs
Once the branch is identified, the routing is straightforward.
Key or transponder fault. A locksmith or dealer. No ECU work, and anyone quoting you for module programming on these facts is quoting the wrong job.
Antenna ring or wiring. A parts-and-labour repair. A new ring is often a plug-in part; on some platforms it needs coding afterwards, which is a session.
Module replaced, needs marrying to the car. An OE programming session on one of the eight platforms on our flash page— $100 for GM, $150 for Ford/Lincoln, Nissan/Infiniti and Mazda, $250 for Toyota/Lexus, Volvo, VW/Audi and Mopar. The manufacturer's own application performs the immobiliser configuration and key registration as published procedures.
Immobiliser data removed from a controller binary. An IMMO-OFF file service at $100 on a supported family, with the family list and its per-operation status published on our coverage matrix— 20 variants, each with separate IMMO-OFF and checksum status rather than one blanket claim. What changes about the car afterwards is worth knowing before you commit: our after-IMMO-OFF post is explicit that removing the immobiliser removes a theft deterrent, and that this is a decision rather than an upgrade.
Controller will not power up or communicate at all. Bench work. Mail-in at $250, or post it to the specialist network free of charge if it is outside what we carry.
What the standards say about access
Immobiliser functions are security-relevant, and the legitimate access routes are documented rather than improvised. SAE J2534 defines the pass-thru interface, SAE J2012 standardises the code definitions your scanner reports, and NASTF administers the secure-access registry that credentialed immobiliser functions run through. Vehicle theft remains a significant enough problem that the Insurance Institute for Highway Safety tracks it as a standing research topic — which is the context for why these systems are deliberately awkward to work around, and why proof of ownership is a normal request rather than an insult.
We do not provide emissions defeat, delete files, or services intended to bypass emissions laws. Our services exist for tuning preparation, diagnostics, repair, motorsport, and off-road use where legally permitted. Customer is responsible for confirming legal use in their location and application.
What a useful booking note contains
Five facts sort this job before anyone touches the car, and all five are things you already know by the time you have finished the scan.
The VIN. It settles the platform, the model year and the immobiliser generation in one string, and it decides which of the eight applications on our flash page is the right one.
Crank or no crank. The single highest-value observation in the whole diagnosis, and the one most often left out of a first message.
Every code from every module— not just the engine controller. A body code alongside a powertrain code changes the branch completely.
How many keys exist, and whether a second one behaves differently. A car that starts on one key and not the other has already told you the answer.
What changed immediately before. A battery, a disconnected harness, a used module, a locksmith visit, an attempted flash. Something nearly always preceded it, and saying so is faster than us finding it.
The short version
Crank or no crank. Try the other key. Scan every module, not just the engine. Say what changed. Those four steps sort almost every security-light no-start into a branch, and three of the branches are not ECU work at all. When it is ours, the price is posted before you commit — a session from $100, a file from $100, bench from $250 — and if it turns out to be a key, we will tell you that instead of selling you a file.