OBD-II Diagnostic Trouble Code
P1191 — Cylinder 6 Glow Plug Circuit Performance
P1191 is a manufacturer-specific code that most commonly indicates the cylinder 6 glow plug circuit is out of its expected current or resistance range during the glow cycle. It normally points to the plug or wiring on cylinder 6. Exact meaning can vary by OEM.
Severity 2/5
Truck-relevant
Service soon
Severity
Service soon
Cylinder 6 Glow Plug Circuit Performance
Code
P1191
Category
Powertrain
Subsystem
Cold Start
Truck-relevant
Yes
What this means for truckers
Service this week on a Cummins X15, Detroit DD15, PACCAR MX-13. Not an immediate stop, but ignoring it usually leads to a more expensive fix downstream.
Before you replace parts — try this first
P1191 on the Cold Start system is most often cylinder 6 glow plug failed open or shorted — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
- Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
- Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
- Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
Extended cranking in the cold, white smoke and a rough idle after cold start, a misfire on cylinder 6 until warm, and a check-engine light. The truck normally drives fine once at operating temperature.
Common causes
- CommonCylinder 6 glow plug failed open or shorted ($90–$350)
- CommonHeat-damaged or corroded connector at cylinder 6 ($60–$250)
- OccasionalChafed or burned feed wire to cylinder 6 ($120–$450)
- OccasionalGlow plug control module output fault ($250–$700)
- RareECM fault ($150–$1500)
How to fix
Measure cylinder 6 glow plug resistance and compare to the others
When to use: First step
Inspect the connector and wiring for heat damage and verify voltage during glow
When to use: When the plug tests good
Replace the cylinder 6 glow plug
When to use: When the plug is out of spec
Test or replace the glow plug control module
When to use: When there is no proper output to a good plug
Affects most
No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.
Related
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P1101 — Mass Airflow Sensor Out of Self Test Range
- → P1102 — Mass Airflow Sensor In Range But Lower Than Expected
- → P1103 — Mass Airflow Sensor In Range But Higher Than Expected
- → P110A — Mass Air Flow Sensor Performance
- → P111A — MAF Sensor B Performance
Frequently Asked Questions
Can I drive with code P1191?
P1191 is severity 2 — service this week. Not an immediate stop, but ignoring it usually compounds the problem. Most common cause for this code is a worn sensor or minor performance fault that won't strand you today but will get more expensive the longer you wait.
How much does it cost to fix P1191 on a commercial truck?
Total repair cost ranges $60–$1500 depending on the root cause. The cheapest fix is usually the first one listed below — Cylinder 6 glow plug failed open or shorted — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will P1191 fail a DOT inspection?
P1191 is generally not a direct DOT inspection fail item by itself. Inspectors focus on safety-critical systems (brakes, steering, tires, lights, hours-of-service). However, the check engine light alone can trigger additional inspection scrutiny. Best practice: fix the code and clear it before a planned inspection.
What's the difference between P1191 and related codes?
P1191 is closely related to: P1101 (Mass Airflow Sensor Out of Self Test Range), P1102 (Mass Airflow Sensor In Range But Lower Than Expected), P1103 (Mass Airflow Sensor In Range But Higher Than Expected), P110A (Mass Air Flow Sensor Performance). These codes share the same subsystem (cold-start) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only P1191 is set without the companions, the issue is more isolated.
How do I prevent P1191 from coming back after a repair?
For P1191 (Cylinder 6 Glow Plug Circuit Performance), prevention centers on the cold start system. Test glow plug system annually before winter. Use a battery in good condition — weak batteries make glow plugs work harder. Park indoors below -10°F if possible.