P0673 – Cylinder 3 Glow Plug Circuit

System: Powertrain | Standard: ISO/SAE Controlled | Fault type: Circuit | Location: Cylinder 3

Definition source: SAE J2012/J2012DA (industry standard)

DTC P0673 indicates the powertrain control system has detected a fault in the glow plug circuit for cylinder 3. Glow plug circuits are used on certain engines to support cold starting and combustion stability by heating the combustion chamber area prior to, and sometimes shortly after, start-up. This DTC is about the electrical circuit behavior, not a guaranteed failed glow plug or a confirmed mechanical engine problem. The exact monitoring strategy, the way cylinder numbering is defined, and the conditions required to set the code can vary by vehicle, so confirm connector locations, pin functions, and test specifications using the appropriate service information before repairs.

What Does P0673 Mean?

P0673 means the control module has identified a fault in the cylinder 3 glow plug circuit. Per SAE J2012 DTC structure conventions, the code points to a specific monitored electrical circuit associated with the glow plug for that cylinder. In practice, the module is expecting the cylinder 3 glow plug circuit to respond in a predictable way when commanded on and off; when the observed circuit condition does not match what the module considers valid, it stores P0673. This definition does not, by itself, prove the glow plug is bad; it indicates the circuit for that glow plug requires electrical diagnosis.

Quick Reference

• Subsystem: Cylinder 3 glow plug circuit (glow plug, harness, connector(s), and any control element used to drive that plug).
• Common triggers: Open circuit, short to power, short to ground, excessive resistance, poor terminal contact, or a driver/output control issue affecting the cylinder 3 glow plug circuit.
• Likely root-cause buckets: Wiring/connector faults, glow plug fault, power/ground distribution issue, glow plug control output/driver issue, control module logic/monitoring considerations (varies by vehicle).
• Severity: Usually moderate; may be most noticeable during cold starts, with potential for hard starting or rough initial running depending on ambient temperature and engine design.
• First checks: Verify cylinder 3 identification, inspect connectors and harness routing, check for corrosion/loose pins, confirm power and ground paths, and look for related glow plug or power supply DTCs.
• Common mistakes: Replacing the glow plug without confirming circuit integrity; misidentifying cylinder numbering; skipping connector pin-fit checks; ignoring power/ground or control-output faults.

Theory of Operation

The glow plug system heats one glow plug per cylinder (or per combustion location) to assist ignition stability when the engine is cold. A control module commands glow plug operation through a relay, module, or integrated driver stage (design varies by vehicle). When commanded, current flows from a power feed through the glow plug heating element and returns through an intended ground path or controlled driver path. The module may monitor the circuit indirectly by current sensing, voltage sensing, or driver feedback to determine whether each glow plug circuit is operating within expected electrical behavior.

P0673 sets when the monitored response for the cylinder 3 glow plug circuit indicates an abnormal electrical condition during a self-test or commanded glow event. Depending on design, the module may detect missing current flow, unexpected feedback, or an implausible circuit state. Because monitoring methods differ, follow service information to identify the specific test points and the expected behavior for the cylinder 3 circuit.

Symptoms

• Hard start during cold ambient temperatures, especially after an overnight soak.
• Extended cranking before the engine starts, more noticeable when cold.
• Rough idle or unstable combustion for the first moments after start-up.
• Misfire-like shaking or uneven running shortly after starting (typically improves as the engine warms).
• Increased smoke on cold start due to incomplete combustion (severity varies by vehicle).
• Reduced efficiency during warm-up, with a brief period of poorer throttle response.
• Warning light illuminated and stored code P0673 in memory.

Common Causes

• Open or high-resistance wiring in the cylinder 3 glow plug circuit (damaged conductor, corrosion inside insulation, or broken strand)
• Poor connector contact at the glow plug, glow plug harness, or intermediate connector (spread terminals, poor pin fit, contamination, partial disengagement)
• Short to ground or short to power in the cylinder 3 glow plug control/feed circuit (chafed harness or melted insulation)
• Glow plug with an internal electrical fault (open element or abnormal resistance) affecting circuit behavior
• Glow plug control module/driver fault for the cylinder 3 channel (if the design uses an external driver module)
• Power supply issue feeding the glow plug system (fuse, fusible link, relay, or power distribution connection with excessive resistance)
• Ground path problem shared by the glow plug module/relay or engine harness (loose ground fastener, corrosion, or high resistance)
• Water intrusion or heat damage in the harness routing near the engine, leading to intermittent opens/shorts under vibration or temperature change

Diagnosis Steps

Tools you’ll typically need include a scan tool capable of reading freeze-frame and live data, a digital multimeter, and back-probing test leads. A wiring diagram and connector pinout from service information are essential because circuit layout varies by vehicle. If available, a current clamp and a battery charger/maintainer help stabilize testing during extended glow plug operation.

1. Confirm DTC P0673 is present and record freeze-frame data and any related DTCs. Address power supply or module communication codes first because they can affect glow plug monitoring.
2. Clear codes and perform a cold-start or commanded preheat event (varies by vehicle) while logging relevant live data. Note whether P0673 resets immediately, only during preheat, or intermittently.
3. Perform a visual inspection of the cylinder 3 glow plug area and harness routing. Look for rubbing, melted insulation, oil saturation, broken clips, or connectors that are not fully seated.
4. Identify cylinder 3 and verify you are testing the correct glow plug and connector. Cylinder numbering and glow plug access points vary by vehicle; use service information to avoid swapping components by mistake.
5. Key off, disconnect the cylinder 3 glow plug connector, and inspect terminals on both sides for corrosion, discoloration, pushed-back pins, or loss of tension. Correct any connector integrity issues before deeper electrical testing.
6. Check the glow plug electrically with a multimeter per service information (resistance/continuity checks). Compare results to the specified range for the application and, if appropriate, compare to another cylinder’s glow plug as a sanity check.
7. Test the cylinder 3 glow plug circuit for opens/high resistance. Using the wiring diagram, perform continuity checks from the driver/module side to the glow plug connector, then repeat while performing a wiggle test along the harness and at connectors to uncover intermittent faults.
8. Check for shorts to ground and shorts to power on the cylinder 3 glow plug circuit with the connector disconnected (both ends if practical). If a short is found, isolate by disconnecting intermediate connectors and rechecking to narrow the affected harness segment.
9. Verify power and ground integrity for the glow plug control module/relay (as applicable). Use voltage-drop testing across power feeds, relay contacts, and grounds during an active preheat event or commanded output; excessive drop indicates resistance in the path that can distort circuit monitoring.
10. If the scan tool supports output controls, command glow plug operation and observe whether the cylinder 3 channel behaves differently than other cylinders (where data is available). If wiring and the glow plug test good but the cylinder 3 command/feedback is abnormal, suspect a driver/module fault consistent with service information.
11. After repairs, clear codes and repeat the same operating conditions that originally set P0673 (especially cold conditions). Confirm the monitor completes and that the DTC does not return, then recheck for pending codes.

Professional tip: Intermittent glow plug circuit faults are often connector- or harness-related and may only appear with engine vibration or heat soak. When live-data logging is available, reproduce the fault by gently moving the harness and connectors during a commanded preheat event, then immediately follow up with targeted voltage-drop tests; this approach is usually faster than replacing parts based on resistance checks alone.

Possible Fixes & Repair Costs

Repair cost for P0673 can vary widely because it depends on what testing confirms as the actual circuit fault, how accessible cylinder 3 components are, and whether wiring repair or component replacement is required.

• Repair or replace damaged wiring in the cylinder 3 glow plug circuit (chafed insulation, broken conductor, melted section)
• Clean, secure, or replace affected connectors/terminals (corrosion, loose pin fit, pushed-out terminals)
• Replace the cylinder 3 glow plug if testing confirms it is electrically out of specification or internally open/shorted
• Repair power feed or ground path issues affecting the glow plug circuit (including high-resistance connections found by voltage-drop testing)
• Repair or replace the affected glow plug control output path if the driver circuit is confirmed faulty (varies by vehicle design)
• After repair, clear the DTC and perform a verification run/monitor reset per service information to confirm the circuit fault is resolved

Can I Still Drive With P0673?

Usually you can drive with P0673, but expect harder starting, rough running, or extra smoke during cold starts because cylinder 3 may not receive proper glow plug heating. If the engine won’t start reliably, stalls, or you have other warnings indicating reduced power or a safety-related issue, do not drive and diagnose the circuit fault first. If you must drive, avoid repeated extended cranking and address the problem promptly, especially in cold conditions.

What Happens If You Ignore P0673?

Ignoring P0673 can lead to worsening cold-start performance, longer crank times, rough idle immediately after startup, and increased emissions during warm-up. Repeated hard starts can place extra load on the battery and starter, and an unresolved circuit problem may progress (for example, increasing resistance at a connector) and become more intermittent or more difficult to diagnose.

Last updated: March 7, 2026

Vehicles Commonly Affected by P0673

• Diesel engines equipped with individual cylinder glow plugs
• Light-duty trucks and vans with glow plug preheat systems
• Passenger vehicles with diesel cold-start assist systems
• Applications using a glow plug control module or integrated driver electronics
• Vehicles operated in cold climates where glow plug use is frequent
• High-mileage vehicles where harness insulation and connector tension may degrade
• Vehicles with recent engine service where connectors may be left loose or wiring disturbed
• Vehicles exposed to moisture, corrosion, or under-hood contamination affecting electrical connections

For the most reliable result, confirm the repair by commanding the glow plug circuit on when supported, logging relevant data, and ensuring the cylinder 3 circuit remains stable under load during warm-up and repeated key cycles.