This checklist covers the standard preventive maintenance tasks for fixed-point CO sensor systems using electrochemical detection technology.
⚠️ Disclaimer: These tasks are guidelines only. They do not include lockout/tagout (LOTO), energy isolation, or other safety requirements. Review and verify suitability for your specific equipment and application. Add all required safety procedures per your company's policies and regulatory requirements before use. You are responsible for the safe and appropriate execution of all maintenance activities.
CO sensors don't fail loudly. The electrochemical cell inside drifts — slowly, quietly — until the sensor is reading 40 ppm when the air is clean, or reading zero when it shouldn't be. Nobody knows until someone runs a bump test, or worse, until someone is already sick.
The fix is not complicated. Consistent bump testing, calibration on schedule, and cell replacement before the end of service life. These are not advanced reliability concepts. They are the minimum.
For the program-level view of what makes sensor PMs actually work, see industrial sensor and instrumentation preventive maintenance.
How to Use This Checklist
Record findings with specificity. "Bump test passed" tells you nothing you can trend. "Bump test: sensor responded to 50 ppm CO in N2, alarm activated at 25 ppm setpoint, response time 18 seconds" — that is a maintenance record. Track pre- and post-calibration readings every time. A sensor that needs larger corrections on each calibration cycle is telling you the cell is dying. That information only exists if you wrote the numbers down the last three times.
Visual Inspection Tasks
Field Checklist — Critical Tasks
| Task | Freq | Type |
|---|---|---|
| Inspect sensor housing for physical damage, corrosion, cracks, or contamination. Ensure cover/lid is secure and sealing properly. | Every PM | MEC |
| Check sensor mounting — confirm it is securely fastened and has not shifted or been accidentally repositioned since last PM. | Every PM | MEC |
| Confirm sensor location is appropriate: installed in the breathing zone, away from direct airflow vents, and per the site CO monitoring plan. | Monthly | ALL |
| Clean sensor face and surrounding area of dust, grease, or debris using a clean dry cloth or low-pressure air. Do not use solvents directly on the sensor. | Quarterly | MEC |
Reference Checklist — Full Task Library
| Task | Freq | Type |
|---|---|---|
| Inspect sensor housing for physical damage, corrosion, impact damage, or moisture intrusion. Confirm enclosure rating is intact (IP/NEMA rating) and cover hardware is secure. | Every PM | MEC |
| Confirm sensor mounting position and orientation per the site CO monitoring plan — verify it has not been relocated, obstructed, or repositioned since last PM. | Every PM | MEC |
| Clean sensor face and surrounding area per manufacturer procedure. Use dry compressed air or a lint-free cloth — no solvents near sensor elements. | Quarterly | MEC |
| Inspect conduit, cable glands, and conduit seals for physical integrity and moisture exclusion. Replace any degraded sealing components. | Quarterly | ELE |
Operational Checks
Field Checklist — Critical Tasks
| Task | Freq | Type |
|---|---|---|
| Verify sensor power indicator or status LED is lit and displaying normal operation. Investigate any fault or alarm indicators immediately. | Every PM | ELE |
| Perform bump test using a certified CO span gas (typically 50–100 ppm CO). Verify sensor responds and alarm triggers within manufacturer-specified limits. Document result. | Monthly | ELE |
| Verify sensor alarm setpoints (low/high) match site requirements and have not been inadvertently changed. Document current setpoints. | Quarterly | ELE |
Reference Checklist — Full Task Library
| Task | Freq | Type |
|---|---|---|
| Verify power indicator and status LEDs reflect normal operation. Check for any fault codes, alarm conditions, or relay output failures and investigate before clearing. | Every PM | ELE |
| Perform bump test with certified CO calibration gas (e.g., 50 ppm or 100 ppm CO in N2). Confirm sensor responds within manufacturer response time spec and alarm relay activates at setpoint. Document pass/fail result. | Monthly | ELE |
| Verify alarm setpoints (low/high) at both the sensor and controller/panel match site-approved values. Document current setpoints. Flag any unauthorized changes. | Monthly | ELE |
| Verify sensor reading against a calibrated portable CO meter placed at the same location. Readings should agree within ±5 ppm or per site acceptance criteria. | Quarterly | ELE |
| Test alarm relay output and all connected downstream functions (horns, strobes, HVAC interlocks, ventilation fans). Confirm actuation and reset properly. | Quarterly | ELE |
| Review sensor response time performance — apply span gas and time the response to 90% of full-scale reading. Compare to manufacturer spec. Flag slow response for sensor replacement. | Annually | ELE |
| Verify sensor communication to controller or building management system (BMS): check signal reading, 4–20 mA output accuracy (if applicable), and any digital protocol status. | Annually | ELE |
Electrical Inspection
Field Checklist — Critical Tasks
| Task | Freq | Type |
|---|---|---|
| Inspect wiring connections and conduit entries for looseness, damage, or moisture ingress. Tighten any loose terminals. | Quarterly | ELE |
Reference Checklist — Full Task Library
| Task | Freq | Type |
|---|---|---|
| Inspect all wiring at sensor terminals, junction boxes, and controller interface for looseness, corrosion, or insulation damage. Torque terminals to spec. | Monthly | ELE |
| Inspect and test the manual test/calibration port (if equipped) for obstruction or damage. Verify calibration gas flow rate is correct per manufacturer spec. | Annually | ELE |
Calibration Tasks
Field Checklist — Critical Tasks
| Task | Freq | Type |
|---|---|---|
| Perform full-span calibration using certified CO calibration gas per manufacturer procedure. Record pre- and post-calibration readings and adjust as needed. | Annually | ELE |
Reference Checklist — Full Task Library
| Task | Freq | Type |
|---|---|---|
| Perform full-span calibration using traceable NIST-certified CO calibration gas. Record zero and span readings pre- and post-calibration. Document technician, date, and gas lot number. | Semi-Annually | ELE |
| Verify zero-point calibration using clean air or nitrogen. Confirm sensor reads 0 ±2 ppm in fresh air after calibration. | Semi-Annually | ELE |
| Review prior 12 months of calibration records and alarm events. Identify any sensors with recurring drift, slow response, or frequent alarm faults. Flag for replacement or root cause investigation. | Annually | ELE |
Sensor Cell Replacement
Field Checklist — Critical Tasks
| Task | Freq | Type |
|---|---|---|
| Replace sensor cell or entire sensor per manufacturer-specified service life (typically 2–5 years for electrochemical cells). Record replacement date on unit and in CMMS. | Annually | ELE |
Reference Checklist — Full Task Library
| Task | Freq | Type |
|---|---|---|
| Replace electrochemical sensor cell or complete sensor per manufacturer service life (typically 2–5 years). Record part number, serial number, replacement date, and calibration values in CMMS. | Annually | ELE |
| Confirm spare sensor cells or complete replacement sensors are on hand per site inventory requirements. Verify shelf life of spares has not expired. | Annually | ALL |
Failure Modes This Checklist Targets
Electrochemical cell depletion. The sensing element inside a CO sensor has a finite chemical reaction capacity — it consumes itself detecting CO over time, and beyond its service life it simply stops responding accurately regardless of what the CO concentration actually is.
Sensor drift. Even within its rated service life, the electrochemical cell can drift off calibration from heat exposure, humidity cycling, contaminant exposure, or age — producing readings that are consistently high, consistently low, or directionally wrong without triggering any fault indicator.
Alarm setpoint deviation. Setpoints can be changed accidentally during power cycling, firmware updates, or panel resets, and the sensor will continue operating normally while alarming at the wrong concentration — or not at all.
Moisture and contaminant ingress. Water infiltrating the housing corrodes terminals and sensor elements; chemical contaminants in the environment can poison the electrochemical cell and permanently degrade sensitivity.
Loss of alarm relay function. The sensor may continue reading CO accurately while the relay output that drives horns, strobes, or HVAC interlock signals has failed open — meaning the detection happens but the downstream response does not.
Slow response time degradation. An aging electrochemical cell may still reach the correct final reading but take significantly longer to get there — long enough that an alarm that should trigger in seconds takes minutes, which in a CO event is the difference between evacuation and exposure.