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11 Apr 2019


In 1964, Walt Disney created a magical movie about a London governess named Mary Poppins.

I was six years old when I first saw the movie and immediately fell into its spell as I was roughly the same age as the two children in Ms Poppins’ care.

The movie also featured a television actor called Dick Van Dyke who played the part of Bert.

He was of course the chimney sweep, but through song and dance he would join Ms Poppins in the mesmerizing action.

But when he wasn’t a singing, dancing, one-man-band, he had the dirty job of cleaning out chimney pipes.

Burning coal in Edwardian fire-places produced a lot of soot.

Left unchecked it would gradually build up and choke the chimney, and at worst, catch fire.

Soot is that black stuff that also pours out of car exhausts, particularly those with diesel engines.

It is made up of carbon particles smaller than 1000Å or 0.1μm.

Any particle smaller than 10μm is respirable and penetrates to the gas exchange region of the lung beyond the reach of the muco-ciliary escalator.

Breathing soot-laden air is undoubtedly deleterious to health.

In 2001 many countries started to mandate that the soot should be extracted from the exhaust using a device called a diesel particulate filter (DPF).

And it was an error message about my DPF that seemed to be keeping my car in limp-home mode.

When working properly layers of a substance called Cordierite (aka magnesium iron aluminium cyclosilicate) filters out the soot particles

But like any filter doing its job the DPF will eventually fill up and clog.

Most diesel cars have a sensor which measures the pressure difference between the intake and output sides of the DPF.

A rising pressure difference means it’s time to clean the DPF.

In my car that means burning off the soot at 500°C by converting the carbon into, wait for it, CO₂.

Unfortunately for those of us worried about the environment, this process is the complete opposite of photosynthesis.

The combustion of the soot occurs when my car detects that the DPF is becoming clogged.

This is particularly an issue for diesel cars mostly driven around town as my car is programmed to inject fuel into the exhaust only at high speed.

Failure to regenerate the DPF when needed can lead to a costly $4,000 replacement of the part.

So was it my sensor, or more expensively was it my filter that was at fault?

As the sensor is a delicate piece of electronics exposed to hot exhaust gases and is significantly less expensive than the filter, I opted to replace it first.

A genuine DPF sensor was $250 from my dealer, but the sensor is made by Bosch and can also be sourced as an after-market part for $100.

For those of us who are into cloning eBay also sells a Chinese version of the sensor for $13 including delivery.

I am told by my procedural colleagues that key-hole surgery requires great skill.

But the mechanic who replaced my sensor had more dexterity than a neurosurgeon as he was working in a tiny space between the firewall and an engine that was still very hot.

So it was with great anticipation that I started my car to again be met with the check engine light still illuminated.

In the Mary Poppins movie, Ms Andrews would need to pull a hat stand from her duffle bag to get my car going again.

Cor blimey!

Safe motoring,
Doctor Clive Fraser


Published: 11 Apr 2019