E-Paper
Proposed PUC rules could make emission readings harder to fake. What may change at the testing centre?
India’s proposed standards would require secure, digitally signed records of every completed pollution test, whether a vehicle passes or fails.
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A proposed amendment to India’s standards for emission-testing machines would make pollution-under-control (PUC) readings harder to tamper with, while leaving the test’s limits and the pollutants it measures unchanged.
Under a proposed amendment to the Automotive Industry Standard (AIS)-137 (Part 8) – dated October 2026 and made under the Central Motor Vehicles Rules – the gas analysers that test petrol vehicles and the smoke meters that test diesel vehicles would have to sign every result digitally and send it straight to a central government server.
What would the amendment require?
Under the proposal, every gas analyser and smoke meter would have to contain a dedicated security chip, a Trusted Platform Module (TPM 2.0), that generates and stores cryptographic keys. The instrument would sign each result with a secret key held on the chip, and anyone checking the result could tell if the figures had been changed after the test.
Results would go to the Pollution Under Control Certificate (PUCC) server over the machine’s own SIM or eSIM, a SIM built into the device. The testing centre could not use Wi-Fi or its own computer’s internet connection to send them. That computer would be allowed to start tests and print certificates, but it could not reach the instrument’s private signing key or change any measured value. Every valid completed test would have to be signed and sent, whether the vehicle passed or failed.
The amendment would also change how a test is run. Measurement would have to last at least 60 seconds, and the result would be the average of the readings taken in the final 10 seconds. The machine would not display live emission readings during the test.
A test that records oxygen at 5% or more would be rejected and have to be repeated. High oxygen content indicates that air has got into the sample, which can lower the readings.
The instruments would have to detect unauthorised opening, changes to their software and attempts to manipulate readings, and would stop testing and signing until they are reverified. Manufacturers would have to register their equipment and keep records of instruments in use and withdrawn. They would also have to maintain audit logs and report security incidents.
Why would the readings need protecting?
Pollution Under-Control Certification (PUCC) has been flagged earlier.
The Comptroller and Auditor General’s (CAG) audit in Delhi, tabled in April 2025 and covering 2015 to 2020, said that 538,000 of 2.21 million diesel vehicles were certified with no test values recorded. It also found that 108,000 petrol, CNG and LPG vehicles were passed “despite emitting CO/HC beyond the permissible limits”. In 76,865 cases, testing and certification together took one minute. Of the 53,655 fines issued for visibly smoking vehicles between November 2018 and October 2020, 98% went to vehicles that held a valid certificate.
The Centre for Science and Environment (CSE), a Delhi-based research and advocacy group, also surveyed testing centres in the National Capital Region (NCR) in 2018 and reported “fake software, false passes and inappropriate tests”. A 2002 World Bank review described how a reading could be cut “by withdrawing the probe slightly from the exhaust pipe”, which lets outside air dilute the sample.
Several of the proposed requirements appear to plug these manipulations. A signed record of every test, pass or fail, would make a certificate issued without test values harder to produce. And the oxygen rule bears on the probe trick.
What does the system already do?
PUC testing has been partly digital for some years.
On June 6, 2018, the road transport ministry notified rules linking PUC test data to VAHAN, the national vehicle-registration database. In June 2021, it notified a common certificate format for the whole country, which introduced a rejection slip for vehicles that fail.
Testing software developed by the National Informatics Centre (NIC) is in use in 31 states and Union territories, according to the ministry. The system geo-fences testing centres, so that a machine can issue certificates only from its registered location, and records photographs and videos of vehicles during tests. “The test results are updated in Vahan system on real-time basis,” the ministry said.
The amendment would move these safeguards into the testing instrument, which would sign each reading before it leaves the machine.
What does the test measure?
For petrol, CNG and LPG vehicles, a technician pushes a probe into the tailpipe while the engine idles. An analyser reports carbon monoxide (CO), a poisonous gas that forms when fuel burns without enough oxygen, and hydrocarbons (HC), fuel that leaves the engine unburnt.
Newer cars are also tested at “high idle”, with the engine at about 2,500 rpm, where the analyser checks a reading called lambda. Lambda shows whether the engine is burning fuel and air in the right proportion.
Diesel vehicles get a smoke test: with the engine warm, the technician floors the accelerator several times while a smoke meter measures how much of a light beam the exhaust blocks.
India’s Bharat Stage (BS) emission standards cap what a new vehicle may emit in laboratory tests. India moved from BS-IV to BS-VI on April 1, 2020, skipping BS-V.
The PUC limits for BS-VI vehicles were laid down in a notification of November 26, 2019.
BS-VI petrol, CNG and LPG four-wheelers face the same limits as BS-IV ones: 0.3% CO and 200 parts per million of hydrocarbons at idle. The smoke limit for BS-VI diesel four-wheelers is 0.7 per metre, against 1.62 for BS-IV vehicles. The same rule bars the test if a BS-IV or BS-VI vehicle’s on-board diagnostics malfunction lamp is lit after the engine starts, and the owner must bring the vehicle back after repair.
The test does not measure nitrogen oxides (NOx), gases that form at high combustion temperatures and go on to form fine particles and ozone. It does not count the particles in diesel exhaust either. BS-VI cut both sharply for new vehicles.
The Netherlands tests older petrol cars against limits identical to India’s for four-wheelers. A 2022 study by the Dutch research organisation TNO found that none of the high NOx emitters among the cars it tested would have failed.
Newer diesel vehicles have a diesel particulate filter (DPF), which traps soot before it leaves the tailpipe. The European Commission said in a 2025 assessment: “Even in a Euro 4 vehicle equipped with a defective DPF, the exhaust’s opacity may be lower than the instruments’ resolution.”
What happens next?
The October amendment’s timeline is not public. Nor is it known whether existing analysers and smoke meters can be upgraded or will have to be replaced.
