B1491 — Ignition Cylinder Sensor Circuit Failure

B1491 — Ignition Cylinder Sensor Circuit Failure. A fault in the ignition system. Short, gentle driving to a shop is usually possible. Typical causes: faulty ignition control circuit in the ECM, moisture or oil in the spark plug wells, worn spark plugs or wrong plug gap.

body electrical

Is it safe to drive?

Drive with caution Drive with caution and avoid load. Ignition faults cause misfires that quickly overheat and destroy the catalytic converter; if the Check Engine Light flashes, stop and fix it first.

How to diagnose with an OBD scanner

Read the code and freeze frame Connect an OBD2 scanner, confirm B1491 is stored, and save the freeze frame data — it shows engine conditions when the fault was detected.
Check for companion codes and TSBs Related codes often reveal the real root cause. Also check manufacturer technical service bulletins (TSB) for your make and model — many B0xxx issues have factory fixes or software updates. Search by VIN on the manufacturer site or nhtsa.gov.
Inspect wiring and connectors Visually inspect the wiring harness and connectors related to the ignition system for damage, corrosion, chafing or poor contact.
Subsystem check Swap the coil to another cylinder and see if the fault code follows it.
Subsystem check Inspect spark plugs for wear, oil or carbon tracks and check plug gaps.
Verify the repair Clear the code, complete a full drive cycle including the conditions from the freeze frame, and confirm B1491 does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for B1491 (parts and labor, USD, independent shop rates): Repair coil wiring or connector: $80–$250; Replace an ignition coil: $150–$380; Replace spark plugs: $90–$250. Overall, fixing B1491 usually costs between $80 and $380. An accurate diagnosis before buying parts saves money — the most expensive scenario is rarely the first one to check.

Common questions

How do I diagnose B1491 myself?

Start with a scan: confirm B1491, save the freeze frame and note any companion codes. Then inspect the wiring and connectors related to the ignition cylinder sensor circuit, and verify the suspect component against live data before buying parts.

What are the symptoms of B1491?

Drivers most often notice: hard starting, especially in damp weather, Check Engine Light on, increased fuel consumption, misfires under load or at idle.

How much does it cost to fix B1491?

Repair coil wiring or connector — $80–$250; Replace an ignition coil — $150–$380; Replace spark plugs — $90–$250. Exact cost depends on the root cause, your vehicle and local labor rates — a proper diagnosis is the cheapest first step.

Is it safe to drive with B1491?

Drive with caution and avoid load. Ignition faults cause misfires that quickly overheat and destroy the catalytic converter; if the Check Engine Light flashes, stop and fix it first.

What is the most common cause of B1491?

The most common causes are: faulty ignition control circuit in the ECM, moisture or oil in the spark plug wells, worn spark plugs or wrong plug gap.

When the engine control module detects a problem in the ignition cylinder sensor circuit, it logs trouble code B1491. This is a body electrical fault that specifically points to a failure within the ignition system—most commonly a wiring issue or a faulty component that prevents the engine from properly firing the air-fuel mixture. The module does not set B1491 on a single bad reading; it typically requires the fault to appear across multiple drive cycles before confirming the problem and turning on the Check Engine Light.

The ignition cylinder sensor sits within the ignition coil circuit and feeds data back to the engine control unit. Its job is to report the precise timing and position of each cylinder's firing event. If the signal goes missing, falls out of range, or shows a short/open condition, the ECU flags B1491 and often stores a freeze frame snapshot of engine speed, load, temperature, and vehicle speed at that exact moment. That snapshot is valuable—reading it first tells you exactly what conditions triggered the code, which makes diagnosis faster and more targeted.

Common Symptoms of B1491

The Check Engine Light is illuminated on the dashboard
The engine is difficult to start, especially in wet or humid conditions
Cylinder misfires occur either at idle or under acceleration
The vehicle feels sluggish with a noticeable loss of power
The engine stutters or jerks during normal driving
Fuel economy drops noticeably compared to normal driving patterns

What Causes B1491

Damaged wiring or a corroded connector in the ignition coil circuit
Moisture or oil contamination inside the spark plug tube well
Worn or improperly gapped spark plugs
A failed ignition coil or coil pack that no longer delivers proper voltage
An internal fault in the ignition control circuit of the engine control module itself

The most frequent culprits are wiring issues and failed coils. These are also the easiest to inspect and verify before moving on to more complex components.

How to Diagnose B1491

Plug in an OBD2 scanner and pull the stored code. Confirm B1491 is present, then record all available freeze frame data—this tells you engine rpm, load, coolant temperature, and speed when the failure occurred.
Check for any other trouble codes that appeared at the same time. Related codes often expose the true underlying cause or point to a secondary problem.
Research manufacturer technical service bulletins (TSBs) for your specific make and model using your VIN on the manufacturer website or NHTSA.gov. Some B1491 cases have factory-issued software updates or known fixes.
Perform a visual inspection of the ignition coil wiring harness and connectors. Look for chafed insulation, broken wires, corrosion, or loose contacts that could interrupt the signal.
If the coil is suspected, swap it with a known-good coil from another cylinder. If the code moves with the coil, you have found the problem.
Remove and inspect the spark plugs. Check for wear, carbon buildup, oil fouling, or incorrect electrode gap, then reinstall or replace as needed.
Clear the code and complete a full drive cycle that reproduces the freeze frame conditions. If B1491 does not return, the issue was likely intermittent and the repair is complete.

Repair Options and Costs

Address the cheapest and most common problems first to avoid unnecessary spending:

Repair or replace damaged ignition coil wiring or connector: $80–$250
Replace a faulty ignition coil: $150–$380
Replace worn or damaged spark plugs: $90–$250

The total cost to fix B1491 typically falls between $80 and $380 depending on which part is actually at fault. A systematic diagnosis before purchasing parts is essential—an ignition coil is not the first thing to replace when a loose connector could be the root cause.

Can You Drive with B1491?

Drive with caution. An active ignition fault causes cylinder misfires, and unburned fuel exiting the engine overheats the catalytic converter over time. If the Check Engine Light is flashing rather than solid, stop driving immediately and address the problem before it causes costly secondary damage.