An analyser reading is only useful if you do the right thing with it. When CO or the CO/CO₂ ratio comes back out of limits, you have a defined path: confirm it, find the cause, fix it, retest — and if it can't be brought within limits, classify the appliance so nobody is left with a CO risk. This guide walks that path. It's study material; only a Gas Safe registered engineer may carry out the work.

Step 1 — make sure the reading is real

Before chasing a fault, rule out the analyser and the method. Is it in calibration? Did it zero properly in fresh air? Is the probe at the right sampling point and depth, with a clean filter, empty water trap and no leaks diluting the sample? A surprising number of "fails" are a tired sensor or a split hose. Confirm the appliance is at the right test condition and let the reading stabilise.

Step 2 — investigate the likely causes

A high CO/CO₂ ratio means incomplete combustion. Common causes to work through (against the manufacturer's instructions):

Step 3 — rectify, then retest

Put right what you found and retest to BS 7967 to confirm the appliance is now within limits. Note that just after disturbing debris, a reading can be temporarily elevated before settling — run on and re-check. If it now passes, complete commissioning, record the readings (on the Benchmark log for a boiler) and cap the test point.

Step 4 — if it can't be fixed

If combustion can't be brought within limits, contact the manufacturer's technical line for further guidance. Where it still can't be resolved, the appliance must not be left in a dangerous state: classify it under the GIUSP (Immediately Dangerous or At Risk as appropriate), seek permission to turn off, fit a warning notice, record and advise in writing.

Ambient CO and emergencies. CPA also covers checking ambient CO in a room (e.g. after a report of fumes or an alarm) — for which a calculated-CO₂ analyser is fine for CO but an NDIR instrument is needed for ambient CO₂. If there's a smell of fumes, a CO-alarm activation you can't resolve, or anyone feels unwell, make safe and call the National Gas Emergency Service on 0800 111 999.
  1. Verify first: calibration, fresh-air zero, sampling point, filter, water trap, leaks, test condition.
  2. Investigate causes: gas rate/pressure, air supply, flue, burner/injector/heat exchanger, contamination.
  3. Rectify and retest to BS 7967; allow disturbed debris to settle.
  4. Pass: complete commissioning, record on Benchmark, cap the test point.
  5. Can't fix: manufacturer guidance, then classify under the GIUSP — don't leave it dangerous.
  6. Ambient CO: NDIR needed for ambient CO₂.
  7. Fumes / alarm / illness: make safe, call 0800 111 999.

10-Question Mock Test

Click an option to see whether you got it right. Explanations appear instantly — no submitting at the end.

Your score: 0 / 10
Question 1 of 10
A reading comes back out of limits. What's the first thing to do?
Question 2 of 10
A high CO/CO₂ ratio indicates:
Question 3 of 10
Which is a likely cause of poor combustion?
Question 4 of 10
After rectifying a fault, you should:
Question 5 of 10
Just after cleaning, a reading is briefly elevated. You should:
Question 6 of 10
Once the retest passes, you:
Question 7 of 10
Combustion can't be brought within limits despite rectification. You should:
Question 8 of 10
When an appliance is turned off as unsafe, you:
Question 9 of 10
To measure ambient CO₂ in a room (not flue products), you need:
Question 10 of 10
A CO alarm is sounding and an occupant feels unwell. The priority is to:

A fail is a procedure, not a panic. Know the steps cold.

PlumbMate drills the CPA1 decision path — verify, investigate, rectify, retest, classify — with quizzes and spaced repetition mapped to the gas ACS tickets.

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