P0674

What P0674 Means

Code P0674 points to a fault in the glow plug circuit for cylinder 4. The engine management system (DME) monitors current and voltage in each glow plug circuit. If the values fall outside the allowed range, the fault is stored.

Glow plugs heat the combustion chamber of a diesel engine before startup, especially in cold weather. Each plug is driven by the Glow Plug Control Module, which takes commands from the DME. The fault can come from:

• a failed glow plug itself (open or short circuit);
• damaged wiring or a damaged connector;
• a faulty Glow Plug Control Module.

This is an electrical fault with moderate severity: the engine will run, but cold starts and emissions get worse.

Symptoms

• Check Engine light: the malfunction indicator lamp comes on in the instrument cluster.
• Hard cold start: the engine may crank longer or start with difficulty at low temperatures.
• Rough cold idle: after startup the engine may run unevenly, with vibration.
• Higher emissions: incomplete combustion when cold raises the level of harmful substances in the exhaust.

Symptoms show up mainly on cold starts and in cold weather. Once the engine warms up, running may return to normal.

Diagnosis and Repair

Step 1: Read the Fault Codes

Use BMW diagnostic equipment (ISTA, INPA) to read codes from DME memory. Confirm that P0674 is active or stored. Check for related codes pointing to other glow plugs or to a shared circuit problem.

Step 2: Test the Cylinder 4 Glow Plug

Disconnect the connector from the glow plug. Measure the plug's resistance with a multimeter. Resistance should match the manufacturer's spec (usually less than 1 ohm). If resistance is infinite (open) or near zero (short), replace the glow plug.

Step 3: Inspect the Wiring and Connector

Check the wires running to the glow plug for breaks, chafing, oxidation, or damage. Make sure the connector is seated firmly and shows no corrosion or burning. Replace damaged wiring sections or the connector as needed.

Step 4: Check the Glow Plug Control Module

If one glow plug is bad, replace it. If the code returns after replacement, or several glow plugs fail at once, check the Glow Plug Control Module. It may fail to supply voltage to the plug or send a wrong signal. Test the module with ISTA/INPA (activation tests and output signal measurement). If the fault is confirmed, replace the module.

Step 5: Clear the Codes and Verify

After the repair, clear the codes from DME memory and take a test drive that includes a cold start. Confirm that P0674 does not return.

Tools needed: BMW diagnostic software (ISTA, INPA), multimeter, electrical circuit test kit.

Common diagnostic mistakes: replacing the glow plug without checking the wiring and control module; ignoring other glow-plug-related codes; fitting incorrect glow plugs (aftermarket or with the wrong heat rating).

Secondary Problems

Ignoring a faulty cylinder 4 glow plug can lead to the following:

• Hard cold starts: the engine will be difficult to start, especially below freezing, which puts extra load on the starter motor and battery.
• Accelerated engine wear: incomplete combustion when cold dilutes the engine oil with fuel, reduces its lubricating properties, and speeds up wear of the cylinder and piston assembly.
• Emissions problems: more unburned hydrocarbons and soot, which can clog the diesel particulate filter (DPF) and reduce catalytic converter efficiency.
• Cascading faults: a bad glow plug can overload the Glow Plug Control Module, which over time can take out other channels and require replacement of the whole module.

Don't put the repair off, especially before the cold season.