Log #15 - Failed Vehicle Inspection and High CO Readings

Emissions results, previous VCDS faults and the decision to check the exhaust system

8 September 2026

The vehicle inspection station we visited on 8 September.

Rear view of the car during the first exhaust emissions test.

Emissions report dated 8 September: CO 0.46% at idle and 0.55% at fast idle; test failed.

View of the inspection lane from the driver's seat.

First Emissions Test and Vehicle Inspection

The vehicle's inspection was due to expire on 17 September 2026. I booked the appointment for 8 September at 13:30 to leave time to deal with any problems.

We entered the exhaust emissions test lane at 12:44. Carbon monoxide (CO) readings exceeded the limits stated on the report:

Idle: 820 rpm, CO 0.46%; limit 0.30%.
Fast idle: 3,000 rpm, CO 0.55%; limit 0.20%.
CO₂: 14.43% at idle and 14.61% at fast idle.
O₂: 0.88% at idle and 0.64% at fast idle.
HC: 100 ppm at idle and 77 ppm at fast idle.
Lambda: 1.010, within the limits stated on the report.

The vehicle failed the emissions test because of the high CO readings. We then proceeded to the periodic vehicle inspection. Without a valid emissions certificate, the vehicle was classified as having a major defect and failed the inspection. A one-month retest period began.

The initial inspection report was later damaged by moisture inside the car. As a result, I could not recheck the complete list of minor defects against that document.

VCDS History and Suspected Catalyst Deterioration

I had recorded two significant fault codes during the VCDS scan in Log #10:

16514 / P0130: Bank 1 Sensor 1 oxygen (lambda) sensor circuit malfunction.
16804 / P0420: Catalyst system efficiency below threshold.

At the time, the Half-FIS display occasionally showed consumption figures as high as 70–80 L/100 km. After the pre-catalyst Bosch lambda sensor was replaced in Log #13, the displayed consumption returned to normal. The engine warning light did not come back on during the period I monitored it.

The high CO readings at the inspection prompted me to reconsider the earlier P0420 fault. I thought the catalytic converter might have lost efficiency, so I compared original ceramic-substrate converters and metal-substrate alternatives in terms of cost and technical specifications.

Checking Before Replacing

I did not want to replace an expensive catalytic converter without a confirmed diagnosis. First, I decided to have an exhaust specialist check the system for gas leaks.

Outcome

The vehicle failed both the emissions test and the periodic inspection because of the high CO readings. The earlier P0420 code raised suspicion about the catalytic converter, but the exhaust system needed to be checked before replacing any parts.