Car Care: Rebooting works in cars too

A mechanic can detect any faults in with a vehicle if it goes into 'limp' mode.

I had a call recently from a mate who was stranded on the side of the road in his ute, as his vehicle refused to go any faster than around 20km/h.

He said the engine had started fine earlier in the day and he had travelled around 50km with no apparent issues before parking the vehicle up for a couple of hours.

The initial restart was fine but less than a kilometre down the road and after slowing for an intersection, the engine speed refused to get above what he described as basically a fast idle and no amount of acceleration or gear changing made the slightest bit of difference.

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He was afraid to switch the engine off in case it did not restart again. My first question was asking him whether there were any dash warning lights staying on. Answer was no. Next step was getting him to switch the engine off, turn the ignition back on, wait for the appropriate dash warning lights to do their usual self-checks before extinguishing themselves and if all okay, try restarting the engine.

Short answer was the engine started and performed as per normal showing no signs of the fault and he was able to drive home with no further issues.

I suspect the engine had gone into what the motoring technical experts call "limp" mode. This is when a problem is detected by the on-board engine management Electronic Control Module (ECM) or computer, and the engine is allowed to keep running to allow the driver to pull over to a place of safety.

As already covered in a previous Car Care column, checking the dash lights is key in determining whether or not any attempts to restart an engine or continue driving should be made.

Red is the definite go-no-further warning sign while orange is usually an alert that a particular fault has developed but it's okay to drive to a place of repair albeit with restrictions in some cases.

For example, if the airbag warning light (SRS) comes on and stays on, it is telling the driver a problem has been detected and the SRS system has been shut down. In the event of an accident and the occupants are at risk of injury, the airbags would not deploy.

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It's a similar scenario with Anti-Locking Brakes (ABS). The driver will retain normal braking, but in an emergency stop, the wheels could lock and the driver would lose the ability to steer away from potential danger.

In my mate's case, no such out of the ordinary dashboard illumination was noted so the decision to shut down and try restarting the engine was a safe call. But the chances of the same problem recurring in future are, as I told him, fairly high.

For some reason, the fault had automatically corrected itself after the engine management system had been shut down and was then rebooted when the engine was restarted. It all sounds similar to the golden rule when having problems with the home or office computer: "Have you tried switching it off and restarting it?".

In the automotive industry intermittent faults like what happened to the ute are often held in the memory of the ECM unit to help technicians identify a problem and quickly eliminate a host of potential possibilities.

Special equipment is required to connect to the ECM and to delve into its memory. In the case of my mate, a visit to his local service agent quickly identified the problem as being fuel related.

The diagnosing gear was so good it even told the technician how many times the engine had been started since the problem had corrected itself. All very clever stuff which underlines the importance of getting a modern motor vehicle checked out by people with all the correct equipment.

But the key message here is not to assume a problem has miraculously gone away, without getting it checked out properly there is every chance it will return.
And, as is often the case, it will happen at the most inconvenient time.

Do you have a car buying or Car Care question for Jack? Send your question to driven@apn.co.nz