Tech guide · NCT
Failed the NCT on emissions? Read your results
Failing the NCT on emissions is one of the most common questions we get about Imprezas, especially imports and modified cars. This guide explains how the petrol emissions test works, how to read the figures on your NCT report, and what they usually point to on a Subaru.
At a glance
Lambda 0.97–1.03
1.00 is perfect. Below 0.97 the engine is running rich; above 1.03 it is lean or has an air leak.
CO under 0.3%
At about 2,500 rpm the catalyst should keep carbon monoxide below 0.3%.
Arrive hot
Drive for 20–30 minutes before the test so the oil and catalyst are fully warm.
How the test works
The tester warms the engine (oil temperature is printed on your report, typically around 80–90 °C), then a probe in the tailpipe measures the exhaust gas at two engine speeds: low idle (normal tickover) and high idle (around 2,500 rpm). Limits depend on the car’s year, but for a catalyst-equipped petrol car like any WRX or STI they are typically:
| Reading | What it measures | Typical limit | In plain terms |
|---|---|---|---|
| CO (low idle) | Carbon monoxide, % volume | Fail above 0.5% | Unburnt fuel from running rich |
| CO (high idle) | Carbon monoxide at about 2,500 rpm | Fail above 0.3% | Same, under a little load; the catalyst should be working hard here |
| HC (high idle) | Hydrocarbons, ppm | Fail above 200 ppm | Raw fuel or oil getting through: misfires or burning oil |
| Lambda (high idle) | Air-to-fuel balance, calculated from the gases | Must be between 0.97 and 1.03 | 1.00 is perfect. Below 0.97 is rich; above 1.03 is lean or an air leak |
Your report shows each reading next to its limit, with PASS or FAIL beside it. Look at which readings failed and by how much: the pattern tells you a lot.
Lambda at high idle (about 2,500 rpm)
What the readings usually mean
| What failed | Most likely causes on an Impreza |
|---|---|
| High CO and lambda below 0.97 (running rich) | Faulty or lazy front oxygen (lambda) sensor; a remap or ECU that runs rich at idle; a leaking injector or high fuel pressure; a coolant temperature sensor telling the ECU the engine is cold; a decat or failed catalyst |
| Lambda above 1.03 (lean, or extra air) | An exhaust leak before the oxygen sensor (common at the up-pipe and downpipe joints), which lets air in and confuses the reading; a vacuum or boost pipe leak; a misfire (unburnt air and fuel come through together) |
| High HC | Misfire: worn plugs, a failing coil pack or plug lead, or a sticking injector. Also oil burning: worn valve stem seals, piston rings, or a leaking turbo seal |
| Passes at low idle but fails at high idle | Usually the catalyst: old, damaged, a “sports” cat that is not efficient enough, or not hot enough on the day |
| Readings miles over the limit (for example 5% CO) | Not a fine-tuning problem. Look for a decat, a failed oxygen sensor, or an ECU or map that ignores the sensor at idle |
Subaru-specific things to check
- Where is your catalyst? JDM classic Imprezas have no centre-section catalyst: the main cat is in the up-pipe or downpipe. If the downpipe has been swapped for a decat, the car will not pass. UK and Irish cars and many later models also have a catalyst further back. Check what is actually fitted.
- Remapped cars: a map that is rich at idle or at 2,500 rpm will fail even with a good cat. Ask your mapper for an NCT-friendly idle and high-idle setting.
- Oxygen sensors: they are cheap compared with a retest. On a high-mileage or unknown-history car, the front sensor is often the first suspect.
- Exhaust leaks: listen for ticking on cold start around the turbo, up-pipe and downpipe flanges. Any leak before the sensor ruins the lambda reading.
- Fault codes: read the ECU (OBD on later cars, Select Monitor or a check-connector flash code on older ones). An oxygen sensor or misfire code points straight at the problem.
Before the retest
- Fix the cause first: additives and “a good run” will not fix readings that are well over the limit.
- Change the oil if it is old or fuel-diluted, and fit fresh plugs if they are due.
- Drive for at least 20–30 minutes before the test so the oil and catalyst are fully hot, and arrive warm.
- Keep the idle stable: no cooling fan or air-con cycling if you can help it.
- Bring the old report so you can compare the new figures with the previous ones.
Still stuck? Post your NCT readings in the forum. Members have been through this many times, and the numbers usually tell us where to look.
More guides
Importing a Subaru to Ireland
Subaru Impreza service guide
Buying a used WRX or STI
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