Heater plugs

Q

I hope you can help with a problem with a 2000 Vauxhall Astra 2.0 DTI with 150,000 miles on the clock. We have owned it for 10 years.

At the start of winter 2017, the Astra was becoming more and more difficult to start. I did some testing and found out that the heater timer/relay was faulty, with no output to the plugs. I replaced this with a new Vauxhall unit and all was well.

Over the subsequent summer, however, every now and again the heater plug light wouldn’t come on and the car was again difficult to start. It wasn’t every time – maybe one in every 20 starts – but if the weather was cold, the light didn’t come on and the car would be difficult to start. When it did start eventually, it would chuck out clouds of blue/white diesel smoke, so I assume the heaters weren’t working.

There doesn’t appear to be any pattern to its non-starting. I am not sure if the car had this issue before changing the relay unit or not – I don’t think it had, but my wife claims it was always an issue.

I did some more testing and discovered that, if I disconnected the temperature sensor on the thermostat housing, the heater plugs would work. I assumed the temperature sensor unit was playing up, so replaced it with a new Bosch unit, but it didn’t change anything. I concluded that I was unlikely to have two faulty sensors and started to look elsewhere.

If the wiring from the sensor to relay had gone open circuit, the relay would read it as the sensor being cold and the heaters would kick in, so I ruled out damaged wiring, pushing me back towards the heater relay again. I have disconnected it and checked the terminals, but they are all clean and corrosion-free, I have power to the unit and a good earth to the relay, so now wonder if the unit is faulty or if there is another issue.

According to the Haynes manual, the relay takes its signal from the ECU, but I am not clear where that is. Is it the unit on the A-post in the passenger footwell? I haven’t checked that, so I assume there could be an issue with either corrosion or bad wiring. Everything else works on the car and, despite is age and deteriorating bodywork, it remains the family workhorse.

I do know a good diagnostics expert, but unfortunately he has never had access to the car in its failed state. He has asked me to leave it with him for a few days, but as the car is in use every day, that is highly inconvenient. Have you come across similar issues in the past? James Radley

A

My first check would be to ensure that the battery is holding a good voltage and maintaining it during cranking. If the battery output is poor, the sensors will not operate correctly.

You have not mentioned checking the heater plugs. If one of these is drawing too much current, it may explain why the relay has failed again. Disconnecting the heater plugs and checking each one using an ohmmeter should show up any inconsistencies without the need to remove them. The heater relay is fed from terminals 4 and 34 on the ECU and checking the terminals and continuity of the wiring between the ECU and the relay may show up a problem. According to Autodata, the engine control module (ECM) should be behind the splashguard at the rear of the nearside front wing.

There is also the possibility that the ECU is not acting correctly on the given information and the unit is at fault. If no other problems are found, it may be worth sending the unit for testing to either ecutesting.com or bba-reman.com, although as the vehicle is in daily use I appreciate that this would take around a week, which may not be convenient.