The engine cut out and will not restart: working through it in order

An engine that dies while you are driving and then refuses to restart is frightening out of proportion to how serious it usually is. A large share of these end up being one connector, one relay or one switch — and a fair number can be fixed at the roadside by somebody with a screwdriver and a clear head.

This is the order to work through it in, and the things worth checking before you call anybody.

First, the part that genuinely is dangerous

The repair is usually cheap. The moment is not, and the two are worth separating before anything else here.

When the engine stops, two things you were relying on stop with it. The brake servo is fed by engine vacuum and stores perhaps one or two assisted stops. After those the pedal goes rock hard and needs a genuinely alarming shove for a fraction of the braking you expected. And hydraulic power steering goes at once, so the wheel becomes very heavy exactly as you need to place the car. Nothing has broken: the brakes and the steering both still work, and neither is assisted. (A fully electric steering system keeps working, since its motor runs from the battery — but most of the cars this article was written about do not have one.)

So, in the order they matter:

• Do not pump the brake pedal. Each press spends some of the vacuum you have left. Press once, firmly, and hold.
• Leave it in gear rather than selecting neutral. A stopped engine still gives some braking through the driveline, and you may want the option of a bump start.
• Steer while you still have momentum and get off the carriageway. Diagnose in a lay-by, not in a live lane.

The corpus rates P0339 — an intermittent crankshaft signal — as critical for exactly this reason rather than because the part is expensive, and the crankshaft position sensor is the article about it. A car that has cut out at speed once will do it again, so whatever it turns out to be, it is a recovery job rather than a drive-home-and-see-how-it-goes job.

The three requirements

An engine needs fuel, air and a spark at the right moment. Every cause of a sudden stop is one of those three going missing, so the diagnosis is a matter of finding out which.

Air almost never disappears suddenly. A blocked filter makes an engine gradually gutless; it does not switch one off mid-journey. So in practice you are choosing between fuel and ignition, and you can usually tell which within a couple of minutes.

Start at the instrument panel, before you open anything. Does it light up normally when you turn the key? Does the starter turn the engine over at a healthy speed? And — this catches more people than anyone admits — is there fuel in it? A gauge that has failed reads full for weeks before anybody questions it.

The distinction to draw first:

• The engine cranks normally but will not fire — it is fuel or spark. Keep reading.
• The engine does not crank, or barely turns — that is a battery, an earth, a starter or a security system, and it is a different article.

The shortlist worth checking first

These are the causes that actually turn up, ordered roughly by how often. Several are specific to the Iranian-market cars this was originally written about, and are noted as such — but the shapes generalise, and the first one applies to a great many European cars.

The inertia switch. This is the single most common cause of "it cut out over a bump and now it will not start", and it is not a fault at all. Many cars carry a fuel cut-off switch designed to kill the fuel pump in a collision. A hard enough pothole, a kerb, or occasionally a heavy application of the brakes will trip one, and it stays tripped until somebody pushes it back in by hand. On a 206 it lives under the bonnet, near the top right of the battery, under a red cap. Press it down until it clicks.

Check this before anything else if the engine died just after an impact, however minor. It costs thirty seconds and it is free.

The double relay. A part that combines two relays in one housing and feeds most of what the engine needs — the ECU, the fuel pump, the injectors, the coil, the canister purge valve, the throttle heater and the oxygen sensor heater. If it fails, nothing runs, and no amount of cranking helps. It sits under the fan tray on the 405 and Pars, beside the ECU on the Samand, Peykan and Peugeot RD, and next to the coolant reservoir on the Pride.

Connectors that let go when hot. The main power connector under the dash works loose as it warms, which produces the maddening pattern of a car that dies when hot and restarts once it has cooled. This is markedly worse where an aftermarket alarm has been fitted, because the connectors on those are frequently not to standard and are rarely seated properly.

The fuel pump connector inside the tank, and the pump's power connector — on a 405 it sits by the driver's right foot. Both corrode.

A main fuse. On the Tiba it is the 80 A in the engine-bay box. On the Samand there is a specific nuisance: the holders for the two fuel fuses spread with age until the fuse is no longer gripped, and the fix is to squeeze the holder back together.

A snapped alternator belt or a seized idler — common on the 206. The engine will run briefly on the battery and then die as the ignition system loses voltage, which looks like an ignition fault.

The idle air control valve, when the engine dies as you lift off or come to a stop rather than at a steady speed.

A failing fuel pump. Pumps rarely stop cleanly; they get weak first, so the engine dies under load and restarts once it has cooled.

A leaking MAP sensor gasket — a known one on the Tondar 90 — and, on later 206s, the ECU itself, which has been seen not to recover from reprogramming and to need repair or replacement.

Checking for fuel — safely

This is where the original version of this article gave advice that should not be followed, and it is worth being explicit about why.

Do not pull the hose off the fuel rail and crank the engine. On a fuel-injected car that rail holds pressure — commonly 3 to 4 bar, and it stays pressurised long after the engine stops. Opening it sprays atomised petrol across a hot engine bay, next to an ignition system you are about to energise. Catching it in a bottle does not make this safe. People are seriously burned doing exactly this.

What to do instead, in increasing order of effort:

Listen for the pump. Sit in a quiet place, turn the key to the position before cranking, and listen at the rear of the car — a two-second whirr from the tank means the pump has power and is turning. Note that it runs only briefly to prime; if the engine does not start it switches off, so you get one chance per key cycle. No whirr at all points at the pump, its relay, its fuse, the inertia switch or the wiring — and that is a much more useful answer than most of what follows.

Check the pressure properly, if you have the tools. Most injection systems have a test port on the rail — a Schrader valve under a cap — and a fuel pressure gauge screws onto it without opening anything to the air. This is the correct test and the only one that distinguishes a weak pump from a dead one.

On a carburettor car the risk is lower but not zero: the pump is mechanical, mounted on the head, and delivers at a fraction of a bar. Pulling its outlet and cranking will show you whether it is pumping. Keep it away from the exhaust, do it in the open, and have somebody with a hand on the key.

One piece of advice from the original that is worth repeating: do not habitually run the tank to the bottom. Sediment and rust settle there, and drawing the last of it pulls that straight into the pump pickup.

Checking for spark — safely

Also worth correcting, and the original got this half right.

Do not pull a lead off and hold it near the block to look for a spark. With no plug in the circuit the coil has nowhere to dump its energy at the voltage it expects, so it hunts for somewhere — and on a system with an electronic coil pack that somewhere can be the coil's own insulation or the ECU driver stage. You can convert a cheap fault into an expensive one in one attempt.

The safe version: put a spark plug in the lead, rest its body firmly against bare metal on the engine so it is properly earthed, and crank. You are looking for a fat blue spark, not a thin yellow one. Better still, use an inline spark tester, which does the same job without an exposed electrode.

A note on wasted-spark systems, which most of these cars use: one coil fires two cylinders at once, one on compression and one on exhaust, and the second spark does no work. This is normal and is not a fault. Coil packs come as one unit — where a single failed output means replacing the whole thing — or as two separable halves, where you can replace only the failed half.

HT leads on these systems should be the silicone type of the specified section; the resistance is part of the design, and cheap replacements cause misfires that read as coil failure.

The original article also described a spare terminal on the coil's low-tension side as a capacitor mounting point, with a capacitance figure that does not parse as a unit. It has been left out. If a terminal on your coil is genuinely unused, leave it unused — the part is not supplied with a capacitor and adding one on the strength of a workshop rumour is not a repair.

Putting it together

With fuel and spark each answered yes or no, you have narrowed it to a quarter of the original problem:

• Spark but no fuel — pump, pump relay, fuel fuse, inertia switch, or a blocked pickup.
• Fuel but no spark — coil, crankshaft position sensor, ignition switch, or the ECU's feed.
• Neither — something they share. The double relay, a main fuse, an earth, the ignition switch, an immobiliser. This combination is good news, because it means one common cause rather than two coincidences.
• Both present and it still will not fire — now consider timing. A snapped cam belt gives exactly this: healthy cranking, fuel, spark, and no compression, often with the engine turning over faster and more freely than it should.

If you have a scan tool

Plug it in before you touch anything mechanical. A stored code will not always name the fault, but it narrows the field enormously — and a car that will not start frequently has a very informative code sitting in memory from the moment it died.

Check engine speed while cranking, too. If the scan tool shows no RPM while the starter is turning the engine, the ECU cannot see the crankshaft position sensor — and that alone explains no fuel and no spark at once, because the ECU will not fire either without knowing where the engine is.