451 CDI 54bhp Sensors

54bhp CDI – The Over-Engineered Pain In The Arse

The 45bhp CDI engine was reliable and efficient, however, due to the Euro 5 emissions standards coming into force, smart had to make changes.

They added a DPF (Diesel Particulate Filter) which removes the particulates from the exhaust gas and heats them up to to about 600 degrees celsius and burns them off. Unfortunately, a standard catalytic convertor only gets to about 500 degrees celsius.

The problem is how DPF’s work. They spend most of their time not being hot enough to burn off the particulates. They just sit there collecting the particulates until the DPF is blocked enough to increase the back pressure to a set point. Once the DPF pressure sensor gets to a certain point, the car increases the fuel use of the engine to make the DPF get hotter. The hot DPF burns off the captured particulates and naturally unblocks itself. This is known as regeneration.

Both the pressure sensor and the temperature sensors are essential for the running of the emissions system. If any of these sensors fail, to maintain emissions, the engine power is reduced.

Exhaust Pressure Sensor

If you remove the rear panels from the 451, you’ll see the main pressure sensor body bolted to the back-side of the crash bar on the left hand side of the car. Unbolting the crash bar may give you more room to work.

The sensor has a crude cover and consists of a wiring connection and 2 hose connections.

The hoses enter the bottom of the sensor and are held in place with crimps. These hose clamps have to be levered open and the hose pulled off. The old sensor can then be disconnected and unbolted before a new sensor is fitted in place.

Small Jubilee clips could be used to hold the hoses to the new sensor however, a strong cable tie may be OK.

The sensor is on the right in this diagram. The rubber hoses connect to formed pipes that bolt to the exhaust.

One connects to the far left of the exhaust body…

…and the other connects to the centre of the exhaust body.

There should be no reason to disconnect these pipes but they just unbolt as normal.

These pipes carry the 2 pressures to the pressure sensor which measures the difference between the 2. When the DPF is blocked enough to exceed a set differential pressure, the DPF will regenerate. Expect a fault code between P2078 and P2088 if you are using MB Star. If you are using some cheap generic code reader, you might get P1408.

Temperature Sensors

For the DPF to regenerate, it must reach 600 degrees celsius. When the DPF regeneration begins, the engine over-fuels.

This increases the temperature of the exhaust gasses which it turn increases the temperature of the DPF. When the DPF reaches the desired temperature, the captured particulates are burnt and released. When the pressure equalises, the regeneration stops.

The temperature of the exhaust gasses are monitored by a pair of sensors. The 1st is in the turbo.

The sensor head is held in place by a captive 17mm nut. To remove this you will need to apply a lot of penetrating fluid.

I would also recommend using a proper 17mm pipe spanner instead of a normal spanner. This will increase the amount of surface area that you can apply torque through meaning you are less likely to damage the sensor head bolt or slip off which would round off the hexagonal head. A 17mm socket obviously cannot be used due to the pipe coming from the sensor.

The 2nd sensor is in the back of the exhaust pipe.

The same removal procedures apply.

The electrical connectors can be seen underneath the engine, on the left side, behind the drive shaft.

Changing either of these sensors is as simple as unbolting it, disconnecting the electrical connector and refitting a new sensor that you have purchased from smart. There doesn’t seem to be 3rd party sensors available yet.

OBD Codes

P2031 to P2036 – Turbo or Exhaust Gas Temperature Sensor
P2080 to P2088 – Turbo or Exhaust Gas Temperature Sensor

Sensor Issues

All 3 of these sensors are prone to failure and the continual update of the part numbers shows that they are trying updated variations. I will try to keep an eye on these parts numbers and I will keep updating the part number list. These numbers are in order of release so the last number in each list should be the most current.

Exhaust Temperature Sensor Revisions

A 008 153 25 28
A 000 905 08 32
A 001 905 15 00
A 000 905 17 32
A 000 905 98 05

Turbo Temperature Sensor Revisions

A 008 153 32 28
A 001 905 13 00
A 000 905 15 32
A 000 905 90 05

Pressure Sensor Revisions

A 006 153 95 28
A 642 905 01 00

What Happens When They Fail?

Initially the car goes into a “kind of” safe mode. Normal safe mode severely reduces the power from the engine which normally limits the top speed of the car to about 40mph. When any of these sensors fail, you’ll get a reduction in engine power but the car will still eventually get up to 60mph despite having no power over 2500rpm.

Have the error codes read to determine which sensor is the problem.

Changing The Sensors

Click here for the pressure sensor change

Credits

Thanks to Michael G for the most current temperature sensor part numbers.