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5,314 Posts
Discussion Starter · #1 ·
If your car won't start, the fix can be as simple as replacing the battery, but there are many other potential causes and troubleshooting can be challenging. The following is an overview, intended to help with the diagnosis.

1. Low voltage
Bad battery
If the starter isn't cranking, or there is only a click, first check battery voltage with a digital multimeter (see photo in post 39), or using the OBC hidden functions (see info in post 37).
It needs to be at least 12V for the starter to be able to crank, while full battery voltage is close to 12.6V.
Faulty alternator
The battery may have died because the alternator isn't charging it.
To test the alternator, first start the car by charging the battery, by jump start, or push start. Measure battery voltage with the engine running, it should be close to 14V, or slightly over.
Bad connections
If the battery and alternator are OK, but voltage is low at the starter, there could be a loose, or corroded connection in the electrical system, causing a voltage drop. Check the ground strap under the engine on the passenger side and the battery terminals. The battery safety terminal (BST), located on the positive battery terminal, is designed to cut off power to the engine in case of an accident. If voltage is sufficient at the battery, but low at the underhood jumper terminals, there is a bad connection at the BST. (see photo in post 60)
Parasitic draw
If the engine runs fine when the battery is charged, or after a jump start, but the battery is dead by morning, there could be a short in the system draining the battery overnight. This is referred to as parasitic draw and on the E46 it is usually caused by a faulty final stage resistor (FSR) of the blower motor. Aftermarket equipment is another common source of parasitic draw.

2. Fuel related
Fuel pump
If the starter is cranking, but the engine isn't starting, the most likely issue is the fuel pump. There are a number of ways to verify.
  • listen for the pump to prime when key is turned to position #2
  • check for fuel at the fuel rail at the Schrader valve
  • measure the actual fuel pressure (or lack thereof) at the rail
  • spray starting fluid into the intake, if the engine stars briefly, fuel isn't reaching the engine.
You can also remove the fuel pump cover and whack the pump, this may get it going temporarily. This "method" can come in handy if you're far away from home.
Engine is flooded
The M54 engines flood easily if they are started and run for a short time, like moving from one parking spot to another. If the engine won't start shortly thereafter, the spark plugs might be saturated with fuel, preventing spark. No start at a later time may be due to loss of compression. See Section 6. for more info on that.
No gas in tank
If the engine stalls, or will not start when the gas is under ¼ tank, the fuel pump is weak and should be replaced. The gas tank has two lobes and a weak pump isn't able to transfer fuel from the other lobe. Fuel gauge maybe inaccurate, reading too high.
Bad gas
No start after the car was sitting for a long time. If your car runs rough and/or won't start right after filling up with gas, the fuel might be contaminated.
Fuel filter
A clogged, or restricted fuel filter usually causes engine performance problems, but it can also contribute to no/hard start.
Fuel pressure regulator
On the 325i/330i this is part of the fuel filter, so by replacing the filter, the FPR will also be renewed. On the 323i/328i it's separate from the filter. These rarely fail on the E46, but it's a possibility. No start due to no, or low fuel pressure can be caused by a defective fuel pressure regulator.

3. Starter
If battery voltage is sufficient at the starter's electrical connections and the starter is not cranking, it is most likely defective. Replacing the starter isn't easy, so make sure the diagnosis is correct before you undertake this task.
Suggestions for troubleshooting: remove the airbox and the intake boots to gain access to the starter. Measure voltage on two of the terminals on the starter solenoid. Terminal "B" should have constant battery voltage. Terminal "C" is energized by the ignition switch via EWS module when starting the engine. Remove the wire from this terminal, measure voltage on the wire with the key in start position.

If sufficient battery voltage is present at both terminals and the starter isn't cranking, it is defective. You can also test the starter by jumpering, or "hot wiring" it. This is done by applying direct voltage to some of the terminals. When voltage is applied to terminal "D", the starter motor should spin without cranking the engine. When voltage is applied to terminal "C", the starter should crank the engine. Make sure you have good engine to chassis ground. Sometimes there is a bad connection at the ground strap that is under the engine on the passenger side. The starter can also get stuck. If voltage is sufficient, but the starter only clicks, this may be the case. Tap the starter while a helper attempts to crank the engine. If it is stuck, this may get it going.

4. Switches and sensors
Ignition switch
If there is no power at terminal "C" on the starter with the key in the start position, there is either an EWS problem, or a bad ignition switch. A faulty ignition switch can cause either a no crank/no start, or crank/no start condition. Intermittent no start, or strange electrical symptoms are also typical signs of a failing ignition switch. Wiggle the key as you start the car, that sometimes works if the switch is bad.
Cranks but no start
Check battery voltage at fuse 29 while starting. This is the ignition switch input to the DME. (see photo in post 60). Check battery voltage at pin 15 on the coil harness with the ignition on. (see photo in post 61). Warning lights going off on the dash randomly, or suddenly while cranking is an indication of a faulty ignition switch.
No crank
Check battery voltage on the black/blue wire at the ignition switch connector with the key in the start position. This wire supplies voltage to terminal "C" on the starter via the EWS module while starting. No voltage in any of the tests indicates a defective switch.
Clutch switch
A defective clutch switch, (or not pressing the clutch pedal far enough) will prevent starting. If battery voltage is OK and starter doesn't crank, this is one of the possible issues. To test the switch, bypass it by inserting a jumper wire in its connector. (wires violet/yellow and black/blue) Switch designs vary throughout the years.
Auto (neutral) safety switch
This is the equivalent of the clutch switch on an automatic, it allows the engine to start only in "P" and "N".
The gear selector switch, located on the side of the transmission performs this function.
Move the shifter back and forth a few times before starting, this may enable start, if the switch is faulty.
Crank position sensor
The DME needs the signal from this sensor to fire the plugs. A faulty crank sensor usually throws a code, making diagnosis simple. If there is no code stored for this sensor, but no spark is suspected, check for spark. See post 62 for info on how to check for spark. If there is spark, the crank sensor must be doing it's job. If there is no spark and no crank sensor code, look at the tachometer while cranking. If the sensor works, the needle should read about 200 rpm.
Camshaft position sensor
A defective cam sensor most often results in driveability problems, like stalling, loss of power and rough running, but it can also prevent the engine from starting. This usually causes CEL, too.

5. Fuses, relays and modules
Immobilizer (EWS)
A faulty EWS module, a key that is not coded to the module, or damaged will prevent the engine from starting. There won't be any power at terminal "C" on the starter solenoid, resulting in a no crank condition. If you hear a beep with the key in the ignition and the driver's door open, the EWS is recognizing the key. Trying to start the car with a second key is another simple way of ruling out a no start due to a non working key. A scan with a BMW specific scanner is necessary to detect EWS faults. If your car is equipped with an additional aftermarket alarm system, make sure it is disarmed.
E-box fuse pack

Under the hood on the driver's side where the DME and other electronic components are located, there is a fuse pack, containing 5 fuses. These are related to engine management. Fuse #3 sends power to both fuel pump relay and the crank sensor. (see photo in post 142)
Main fuse panel
Check fuses 9, 22, 27, 29, 30 (ignition switch), 54 (fuel pump), and 14, 49, 67 (EWS). Inspect all fuses in the glove compartment if you're dealing with a hard to diagnose no start problem. Check the high amp fuses above main fuse panel (fuseable links). There is an 80 amp for the DME and DME main relay, and a 50 amp fuse for the ignition switch. Check the harness fuse: behind the carpeting on the passenger side shock tower there is a large fuse. The cable here supplies the relay panel, so if this fuse is blown, almost everything will be out.
DME Relay
Unlike the older models, this isn't problematic on the E46. Test this relay if there is no battery voltage at the E-box fuses. See post 408 for info on trouble shooting!
Electronic Control Unit (DME)
DME failure is also not common, but possible. A scan with a BMW specific scanner usually reveals if the unit is faulty. There may be no codes stored. If there is no spark, or injector signal and all DME inputs and wiring are OK, the DME needs replacing.
Fuel pump relay
No power at the fuel pump may be caused by a bad fuel pump relay. When checking for power at the pump, the engine needs to be cranked. A simple test for the fuel pump relay is to swap it with the horn relay, they are identical. Additional electrical tests can be done by jumpering terminals at the relay board. If the pump runs with the terminals jumpered, but not with the relay in place, the relay is defective.

6. Mechanical issues
No compression
An engine needs spark, fuel and compression to run. If there is both spark and fuel and your car won't start, test engine compression. At times a condition called cylinder wash down occurs. If the engine started and run for a short time, or when an engine is cranked without starting, a lot of fuel is dumped into the cylinders. This can wash the oil off the cylinder walls and lead to loss of compression. Pour some oil in each cylinder, this will restore compression.
Camshaft timing
The timing chain assembly is reliable on the 6 cylinder engines, but can be problematic on the 4 cylinders models.
Engine seized
An internal mechanical failure can cause the engine to lock up. Try to turn the crankshaft manually to see if the engine is frozen.
The crankshaft can not rotate due to liquid in the combustion chamber. This can be fuel, coolant, or oil, caused by a leaking fuel injector, head gasket, or a malfunctioning CCV system. Remove spark plugs and crank the engine if this is suspected.
Vacuum leak
Vacuum leaks usually cause rough running, or misfires, but a large vacuum leak can prevent starting. If your car won't start after working around the intake, this is the most likely problem. In this case the engine may start up momentarily, but stalls immediately.

272 Posts
Re: E46 NO START – A list of possible causes

This should be stickied. Good work OP.


178 Posts
Don't forget about the voltage regulator per Mango's mention @ http://forum.e46fanatics.com/showthread.php?t=923563 : "

Voltage regulator:

The voltage regulator is located on the back of the alternator. It regulates voltage to ensure proper operation of the electrical system. Basically by around 6-10 years, they start to fail. This will kill a new battery. Replace it.This one is a bit tricky because you'll have to pull your alternator out to know which kind you need. There's a rounded D-shaped connector and squared off rectangle connector. Pray you have the squared off rectangle connector because it's cheaper. The D-shape connector is found by searching under the BMW Z4. "

Thanks for posting this as it's great info for all those who don't search first!

5,314 Posts
Discussion Starter · #12 · (Edited)
I appreciate all the positive feedback!

Hi Guys ! if the key has gone bad will that render the car not to start ? I have put in a new battery ! lights work windows etc but she wont fire up !!! thanks Kerri ..and I will keep reading the FAQ
Yes, it can be the key, try a spare key, or scan for codes to find out.

29 Posts
I ordered a plastic spare key today form the dealership! $ 70.00 I needed a spare and it was that or a $ 200 one , a friend is bringing the scanner over tomorrow as well ! I am so hoping its the key , I will let you know the outcome ,

29 Posts
Ok! so it turns out its the starter motor, bloody thing left me stranded in the sun outside chuckee cheeses it did ! My mechanic has it out ! the brushes were worn out ! so he rebuilt it and was about to put it back in when I asked about the solenoid , thats when I stopped the job in its tracks, it appears that a solenoid was unattainable for the danzo motor and he assured me the solenoid was fine and would last ,no problem! He wants $159 for his rebuild and 300.00 for the in out ( which after seeing the extent of the work , I thought was ok ) my issue is the solenoid , I did some checking and a bosch rebuild is only $125. does anybody know if the Bosch rebuild would be a rebuilt or new solenoid , I really want to drive my car and am tempted to let him put the old one back in , but seeing the work involved in changing it again I am thinking I should go the Bosch route ! any input would be appreciated ..I really do love my car !! ..Kerri

12 Posts
Re: E46 NO START – A list of possible causes

Neutral safety switch is a very common Problem for a no crank no start situation second is the crankshaft sensor check for fuel and spark always if it pass than you are into trouble it will be something mechanical

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5 Posts
My E46, 318i, 2002 model wouldn't start (it cranked) but after BMW mechanic connected the diagnosis laptop to my car and re-programmed the EWS/DME (?) the car worked fine. The mechanic tells me that DME might have a problem (I had the EWS changed 2 weeks ago before this issue) and i should change the DME ($1500) please help
Could it be a software synchronization issue between EWS and DME?
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