No-Start Diagnostics: Separating Crank, Fuel, and Spark from Electrical Faults

Goal: Diagnose by Symptom, Not by Parts

A “no-start” complaint can mean three different problems: the starter does not rotate the engine (no-crank), the starter rotates the engine but it won’t run (crank-but-no-start), or it starts sometimes (intermittent). The fastest way to avoid part swapping is to pick the correct pathway first, then do only the minimum electrical checks that prove or disprove each branch.

Throughout this chapter, treat these as your minimum electrical checks (you will repeat them in different places):

• Battery voltage under load (during crank attempt or with a high load applied).
• Key-on voltage at critical points (fuses, relays, coils/injectors, starter trigger).
• Immobilizer/anti-theft indicators (dash light behavior, message center cues) as observational hints.
• Fuse/relay checks (power in/power out, and command vs. output).

Before You Measure: Confirm the Symptom

• No-crank: Key to START, engine does not rotate. You may hear a click, rapid clicks, or silence.
• Crank-but-no-start: Engine rotates at normal speed but never catches.
• Intermittent start: Sometimes no-crank, sometimes crank-no-start, sometimes normal. Treat it as “capture evidence” and test during the failure.

Pathway 1: No-Crank (Starter Does Not Turn)

1) Quick Observations (No Tools)

• Cluster behavior: Do dash lights go very dim when you turn to START? That often points to a high current draw or weak supply.
• Anti-theft/immobilizer lamp: If it flashes rapidly or shows a “key/lock” message, the vehicle may be inhibiting crank or fuel. Note it; don’t assume it’s the only cause.
• Sound clues: Single click (solenoid attempt), rapid clicking (low voltage), silence (no command, open circuit, or inhibited start).

2) Minimum Electrical Checks (In Order)

Check A: Battery Voltage Under Load (During START Attempt)

Measure directly on the battery posts (not the cable ends). Have a helper hold the key in START for 2–3 seconds while you watch the meter.

• If voltage collapses very low during the attempt, you have a supply problem (battery state/health, internal connection, or extreme draw). Next step is to compare posts vs. cable ends and check voltage drop.
• If voltage stays reasonably steady but there is no crank, suspect a control/command issue (fuse/relay, park/neutral switch, clutch switch, ignition switch output, starter relay control, immobilizer inhibit) or an open in the high-current path.

Check B: Battery Posts vs. Cable Ends (Prove Connection Quality)

Still during a START attempt, compare voltage at the battery posts to voltage at the cable ends (touch the metal terminal on the cable, not the post). The goal is to catch hidden resistance at the connection.

• Normal: Post voltage and cable-end voltage are nearly the same.
• Problem: A noticeable difference indicates a poor connection at that terminal (corrosion between terminal and post, loose clamp, damaged terminal).

Check C: Starter Solenoid Trigger Wire Voltage (Is the Starter Being Told to Crank?)

Locate the starter solenoid “S” terminal (small wire). Back-probe it if possible. Measure voltage from the trigger terminal to a known good engine ground while the key is held in START.

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• ~Battery voltage present at the trigger wire but no crank: suspect starter/solenoid fault, poor main power feed to starter, or poor engine ground.
• Low or zero voltage at the trigger wire: suspect start command path (starter relay, ignition switch START output, neutral safety/clutch switch, immobilizer inhibit, wiring).

Check D: Ground Voltage Drop (Starter Case to Battery Negative)

During a START attempt, measure voltage between the starter housing (or engine block near starter) and the battery negative post. This reveals whether the ground path can carry starter current.

• Low reading: ground path is likely OK.
• Higher reading: excessive resistance in ground cable, ground strap, connection to block/body, or corrosion at contact points.

Check E: Fuse/Relay Checks (Power In/Out and Command)

Focus only on the start-related circuits: main fuses feeding the starter relay/ignition switch, and the starter relay itself.

• Starter relay power feed: with key OFF/ON, confirm the relay has battery feed where expected.
• Relay command: with key in START, confirm the relay coil is being commanded (one side powered, the other grounded or vice versa depending on design).
• Relay output: with key in START, confirm voltage leaves the relay toward the starter trigger circuit.

Pathway 2: Crank-But-No-Start (Starter Turns, Engine Won’t Run)

When the engine cranks normally, the high-current starter circuit is usually “good enough.” Now your job is to separate: (1) electrical power/command to fuel and ignition, (2) immobilizer inhibit, and (3) non-electrical causes (fuel pressure/mechanical/timing). This chapter stays on the electrical side: you will verify that the engine management system is being powered and is allowed to operate.

1) Quick Observations

• Tachometer movement while cranking (if equipped): lack of movement can hint at missing crank signal, but don’t jump to sensor replacement—first verify ECU power and grounds and check for blown fuses.
• Immobilizer/anti-theft lamp: many vehicles will crank but disable injectors or spark when anti-theft is active. Note lamp behavior with key ON and during crank.
• Fuel pump prime sound: absence can be electrical (fuse/relay/power) or command-related.

2) Minimum Electrical Checks (In Order)

Check A: Battery Voltage Under Load (During Crank)

Even if it cranks, low cranking voltage can cause modules to reset or coils/injectors to stop operating. Measure at the battery posts during crank.

• If voltage is very low during crank: address supply/connection issues first (posts vs. cable ends, ground drop). A weak supply can mimic fuel/spark problems.
• If voltage is stable: proceed to key-on power distribution checks.

Check B: Key-ON Voltage at Critical Fuses (ECU/IGN/INJ/COIL)

With key ON (engine not running), check for battery voltage at the fuses that feed:

• ECU/PCM power (often labeled ECM/PCM/EFI)
• Ignition coils (COIL/IGN)
• Injectors (INJ)
• Fuel pump/EFI relay feed (F/P, PUMP, EFI)

Do not only check continuity with the fuse removed; confirm power in and power out with the fuse installed (or by probing both test points on top of the fuse).

Check C: Relay Function (EFI/Main Relay and Fuel Pump Relay)

For crank-no-start, the most productive relays to verify are the main/EFI relay (powers engine controls) and the fuel pump relay (powers pump). Minimum checks:

• Key ON: does the main/EFI relay energize and provide output voltage to its loads?
• Cranking: does the fuel pump relay energize when commanded (some vehicles only run pump during crank/after RPM signal)?

Check D: Key-ON Voltage at a Coil or Injector Connector

Pick an easy-to-access coil or injector. With key ON, one terminal is typically a power feed (design varies). Confirm that the expected power feed is present.

• No power at coil/injector: trace back to the related fuse/relay/main power distribution.
• Power present: electrical supply is likely OK; the remaining issue may be control (ECU not pulsing), immobilizer inhibit, or non-electrical fuel/spark generation issues.

Check E: Immobilizer/Anti-Theft as an Observational Gate

Do not guess. Use the dash indicator behavior as a cue to decide whether to prioritize security system diagnosis:

• Indicator shows active fault/inhibit (rapid flashing, key symbol, “immobilizer active” message): expect crank-no-start with no injector pulse or no spark on many models.
• Indicator behaves normally: continue with power/relay/fuse verification and then move to fuel/spark testing methods (outside this chapter’s scope).

Pathway 3: Intermittent Start (Sometimes Starts, Sometimes Not)

Intermittent faults are usually connection-related (loose terminals, corroded grounds, failing relays, worn ignition switch contacts) or condition-related (temperature, vibration). The key is to stop “testing the good moment” and instead set up measurements so you can capture the bad moment.

1) Define the Intermittent Pattern

• Intermittent no-crank: treat like the no-crank pathway, but focus on starter relay control, trigger wire voltage, and voltage drop at connections.
• Intermittent crank-no-start: treat like crank-no-start, but focus on main/EFI relay output, ECU/IGN fuse power, and immobilizer indicator behavior when it fails.

2) Minimum Electrical Checks Designed for Intermittents

Check A: Battery Voltage Under Load During the Failure

When it fails, immediately measure battery voltage at the posts while attempting to crank (or while it cranks but won’t start). Record the number. Intermittents often “self-heal,” so written notes matter.

Check B: Quick “Power Distribution Snapshot” at Key-ON

When the failure is present, do a fast check of key-on voltage at:

• ECU/PCM fuse(s)
• IGN/COIL fuse(s)
• INJ fuse(s)
• Main/EFI relay output (if accessible)

If any of these are missing only during the failure, you’ve proven an electrical distribution problem (relay, ignition switch feed, fusebox connection) rather than a sensor or mechanical issue.

Check C: Relay Swap as a Test (Only After Measuring)

Relays can fail intermittently. A relay swap with an identical known-good relay can be a valid test only after you have evidence that the relay output is missing when commanded. If you swap without measuring, you lose the chance to prove the fault.

Check D: Voltage Drop “Wiggle Test” (Targeted)

During a crank attempt (or with key ON for low-current circuits), gently wiggle:

• Battery terminals and main cables
• Ground straps (battery-to-body, body-to-engine)
• Fusebox connectors
• Starter relay and main relay in their sockets

Watch for sudden changes in voltage readings (posts vs. cable ends, ground drop, relay output). A change indicates a connection or component that is sensitive to movement.

Practical Flowchart: From “Does the Starter Turn?” to Measurements and Actions

START: Key to START position → Observe: Does the starter turn the engine? (crank RPM present?)

├─ NO (No-crank)
│   ├─ Observe anti-theft indicator abnormal?
│   │     └─ Note it (do not stop here). Continue electrical checks.
│   ├─ Measure battery voltage at POSTS during START attempt
│   │     ├─ Voltage drops very low
│   │     │     ├─ Compare voltage: POSTS vs CABLE ENDS during START
│   │     │     │     ├─ Difference found → Clean/repair terminal connection(s)
│   │     │     │     └─ No difference → Measure ground voltage drop:
│   │     │     │           Starter case to battery NEG post during START
│   │     │     │           ├─ High drop → Repair ground cable/strap/connections
│   │     │     │           └─ Low drop → Check starter main feed at starter B+ and starter condition
│   │     │     └─ (After repairs) Re-test crank
│   │     └─ Voltage stays reasonably steady
│   │           ├─ Measure starter solenoid TRIGGER wire voltage during START
│   │           │     ├─ ~Battery voltage present → Check ground drop + main B+ at starter; suspect starter/solenoid
│   │           │     └─ Low/zero voltage → Check starter relay:
│   │           │           - relay power feed (key ON)
│   │           │           - relay command (key START)
│   │           │           - relay output (key START)
│   │           │           If command missing: suspect P/N or clutch switch, ignition switch START output, immobilizer inhibit
│   │           └─ Verify start-related fuses (power in/out)
│   └─ ACTION: Repair proven voltage drop / missing command / failed relay / starter as indicated

└─ YES (Crank-but-no-start)
    ├─ Observe anti-theft indicator abnormal?
    │     ├─ Yes → After confirming ECU/IGN fuses have power, prioritize immobilizer diagnosis
    │     └─ No → Continue
    ├─ Measure battery voltage at POSTS during CRANK
    │     ├─ Very low → Fix supply/connection first (posts vs ends, ground drop)
    │     └─ OK → Continue
    ├─ Key ON: Check voltage at critical fuses (ECU/PCM, IGN/COIL, INJ, FUEL PUMP/EFI)
    │     ├─ Missing power at any → Trace to main/EFI relay, ignition feed, fusebox connections
    │     └─ Power present → Continue
    ├─ Check main/EFI relay output (key ON) and fuel pump relay output/command (key ON and/or CRANK)
    │     ├─ Output missing when commanded → Repair relay/circuit
    │     └─ Output present → Electrical power distribution likely OK; proceed to fuel/spark generation tests (outside this chapter)
    └─ ACTION: Restore missing key-on power/relay output or address immobilizer inhibit if indicated

Measurement Targets Cheat Sheet (What to Probe First)