A little engine management light (EML) came up on our car last September, its a Vauxhall Astra H (2006) 1.6L petrol. As the weather got colder I noticed that the heating wasn’t working too well either. I had always thought ‘those modern cars you can’t do anything with them it’s all computers’. Well, it turns out that much like any other computer, with the aid of the internet you can do loads.

I learnt that you can get the engine management system to tell you its error codes, there are a bunch o’ refernces to the brake pedal test:

Hold down both the brake and accelerator and turn the ignition on (but don’t start the engine), keep the pedals held down for a while and in place of your mileage the display pops up ECN followed by any stored error codes. In my case it showed 011505, appararently you remove the last 2 numbers, leaving a 4 digit code which you can look up (free registration required, some good advice in this forum, but also plenty of gimps).

This code indicaates a coolant temperature sensor error. So I tested the coolant thermostat. At 0°C the resistance was about 5kΩ and dropping as I started the engine and it warmed up. So the sensor was OK.

A little more Googling and someone suggested it was a thermostat stuck open, hence the error; the car knows the thermostat is open and so the temperature should be over 107°C, but the temperature sensor was (accurately) reporting a temperature around 50 – 70°C.

It turns out that a thermostat for a 2006 astra costs £145 +VAT from aVauxhall dealer, a bit cheaper from other sites (part no 24405922 for an 06 Astra H 1.6L), but I needed it quickly. Drain the coolant -keep it for refilling, the coolant drains from a red plastic tap drivers side, bottom of the radiator. Remove the battery (for access), disconnect a few wiring looms and connectors, remove 4 torx head bolts (one has tricky access, I needed to loosen off the manifold heat shield to get at it). The thermostat and its housing come away and replace with a new ‘un.

The EML remains lit for 30? ignition starts after the error has gone and the error code remains in the ECU memory. I was keen to clear the EML light so I knew I’d fixed the problem, I tried disconneting the battery for 4 hours to no effect. Patience won out in the end and the light went out. The error code is still in the memory.

Another handy thing I learnt was that the radio display can show you all sorts of things. With the radio turned on, hold down the Settings button until you hear a bleep. Then press the button with a circle and a dot in the middle next to the time/radio display. After I pressed this 16 times the radio display shows V:0   T:0 – velocity in km/h and coolant temperature in °C. The former handy next time I am in Europe, the latter infomring me that the engine runs around 107°C when all is well and the thermostat seems to switch at about 109°C, sending the coolant around the radiator to cool it further.

It was cold, but the feeling of satisfaction was good.