Car won't start: how OBD-II tells an immobilizer fault from fuel/electrical

“Cranks but won’t fire” — one of the scariest symptoms for a driver. Causes range from “battery flat” to “immobilizer chip fried”. OBD-II can eliminate half the hypotheses in 5 minutes. Below: algorithm from simple to complex.

Step 0. Battery (not OBD)

Before OBD-II, make sure:

• Starter cranks strong? If “clicking” — low battery voltage
• All warning lights bright during crank? If dim — battery

OK, not our topic — move on to OBD.

Step 1. Does the ECU talk

Plug the ELM327 into the OBD port, turn ignition ON (don’t crank). Via Car Diagnostics run a quick scan:

• Got ECU ping? → ECU alive, wiring intact
• No response → either ECU is dead (rare), or the OBD connector has no power (more often — cabin BCM fuse)

Step 2. Read DTCs

Active codes almost hand you the answer:

Immobilizer / key

• P1602, P1603, P1604 — “Immobilizer communication error” — ECU can’t read the key via receiver
• B10CC, B10D0 — “Transponder issue” — key chip itself is dead
• U0300, U0100 — Lost communication with BCM / IMMO module

Symptoms plus these codes:

• Key indicator flashes on start attempt
• Engine starts for 1 second then dies

Fix: key battery (CR2032 — $2), key learn ($30-80 at a shop), immobilizer antenna ($50-150).

Fuel system

• P0087 — Fuel pressure below spec (pump dying)
• P0088 — Fuel pressure above spec (regulator)
• P0230-P0233 — Fuel pump relay
• P0201-P0206 — Injectors

Symptoms plus these codes:

• Cranks but never catches — pump
• Starts and dies in 2-3 sec — pump can’t hold pressure

Fix: pump relay ($10-30), pump itself ($100-400), injector ($60-200).

Position sensors (crank / cam)

• P0335, P0336 — Crankshaft position sensor (without signal ECU can’t start at all)
• P0340, P0341 — Camshaft position sensor
• P0016, P0017 — see timing chain post — stretched chain

Symptoms plus these codes:

• Starter cranks long, sometimes catches, sometimes not
• Engine dies while driving, won’t start until cooled

Fix: crank sensor ($30-100), cam sensor ($30-100), timing chain ($500-1500).

Ignition (petrol)

• P0350-P0358 — Ignition coil misses
• P2300-P2312 — High-voltage coil problems

Symptoms:

• Petrol smell on start attempt (fouls plugs)
• Engine dies when trying to move off

Fix: plugs ($30-80), coils ($60-150 each).

ECU / software

• P0605 — Internal ECU error (program corrupted)
• P0630 — VIN not programmed (after ECU replacement)
• P0603 — ECU memory loss (battery went flat)

Fix: ECU reflash at a specialist shop ($100-500).

Step 3. Read Live parameters during crank

If DTCs said nothing — enable Live Monitor and crank the starter. Watch:

• RPM during crank — if 0, crank sensor dead; if 200-300 and doesn’t rise — sensor OK, problem is further downstream
• Voltage (0x42) — should hold >10.5V. Drop to 8V → battery collapses under load
• Fuel pressure (if available) — should be ≥ 45 psi for petrol, ≥ 4400 psi for common-rail diesel
• Coolant temp — should show a sensible temperature

Step 4. Cross-check with symptoms

Starter	Fuel prime	Symptom	Hypothesis
Cranks	Yes	Won’t start	Immobilizer, ECU, sensors
Cranks	No	Won’t start	Pump, relay
Clicks	—	—	Battery, starter
No crank	—	—	Battery, ignition switch

Quick checks

• Ignition ON, listen 2-3 sec — you should hear the fuel-pump prime buzz
• Smells like petrol — plugs flooded, crank starter with throttle floored (open-loop mode — should burn off)
• Key removes easily — immobilizer accepted. Won’t remove = ignition switch stuck

Our Chat Agent answers “won’t start” as a dialog — asks clarifying questions and narrows the cause from DTCs + symptoms.

Try it on the free tier — pair an ELM327 adapter, at first start attempt Car Diagnostics reads all codes and shows hypotheses.