P0420 AGAIN!

This fault code seems to be cropping up more and more, by that I mean it comes back after the fix. But why?
Firstly lets understand the fault code and how its generated.
Modern cars have 2 O2 sensors, the first before the cat, monitors the oxygen content of the exhaust gas. It uses this to adapt the fuelling via the fuel trim to keep the mixture within acceptable limits. The 2nd sensor after the cat monitors the efficiency of the cat. If the switching ratio of the O2 sensor is similar then the cat isn't working. This ratio is monitored and once a set ratio is exceeded the fault code P0420 is set.
So why does this fault code come back?
There are 2 causes poor quality replacement parts, and not fixing underlying causes.
If you are presented with this code ensure before fitting a new cat the reason for the failure is correctly identified and fixed. Missfires will make short work of a new cat.
Once the cause is rectified ensure the replacement is of OEM quality, too often the returning code is down to the quality of the replacement parts.

Rally car trouble

This week we have a rally car to deal with. Once warm it cuts out, will not re-start until cold again. A simple temperature related problem so why has it proved so difficult to fix?
time to investigate.

Turns out a relay is failing.

No Codes No Fix

A local garage a had a few problems with this one.

The car cut out, and will not restart.

If left for 5 mins or 5 hours it will restart but cuts out then won't start again.

They tried to scan it for codes, but this resulted in the dreaded no codes found.

The vehicle did not support EOBD so no pending codes or freeze frame data.

This is where you need to have a basic understanding of the control systems.A few quick checks, and hey presto the Crank Angle Sensor is found to be faulty.

Once again checking inputs and outputs with a scope provides the answers when scan tools fail.

A new sensor restored normal operation.

No fault codes what next?

Technicians are becoming more and more dependant upon fault codes. This is due to the increasing complexity of modern vehicles.

But how do you tackle diagnostic problems without codes.

The first example this week was a non starting Clio 1.4 single point injection.

No communication with the scan tool thus no codes.

Where do you start with this kind of problem?

One of my first tests is power to the injectors, this is a good idication of ECU, main engine relay, & earth are all ok.

In this case we had no power at the injector. The reason is proberly the cause of the non start.

Using the Autodata wiring diagram shown it is possible to work out where the feed comes from. The fuel pump relay, this checked out OK, so the main engine relay was tested, I tend to bridge them as a quick test and after this not only did the engine start but communication with the scanner was restored. Problem solved or was it, I haven't proved how it had failed. It turns out the switched earth signal from the ECU has gone missing, the wiring had gone Open Circuit.

A quick repair restored normal operation, but why had a new ignition coil been fitted in an attempt to get this vehicle going?

The garage had tested for a spark during cranking, not being able to gather codes resulted in a guess. The guess was based on previous experience of this system where coils had been known to fail.

Following a TESTING procedure that I have refined over years of diagnostic testing prevents costly mistakes.

Nissan Touring car Missfire

Interesting one this, missfire complaint on a Nissan Primera.

No big shakes apart from the fact it is a 2000 super tourer.

A 2.5 l 320 BHP N/A engine.

But engines are engines and they all need fuel, spark and compression.

The visual inspection found a bad connector, on one of the eight yes eight injectors.

Once repaired and shaken down on the track all was good.

It is now racing and winning in the Welsh Saloon Car Championship.

For more details check out WRDA.co.uk

ABS trouble

Most ABS faults relate to the wheel speed sensors, the wiring or the physical condition of the system.
This Ford Puma was no exception, the pedal pulsed when driving, the ABS warning light was illuminated, and it was clear the vehicle had sustained some damage recently.
A quick look at live data showed one wheel speed sensor dropping out occasionaly, but why?
No need for scopes here the front left wheel speed sensor wiring had been damaged but not repaired when the panels had been replaced after a minor RTA. New sensors are expensive so a repair to the wiring loom was the option taken by the customer.

Latest cars in the workshop

We looked at a Ford Cosworth Race Car just before it went to Brands Hatch on the trailer to race for the weekend.
Once fired up it was obvious there was something wrong.
Not all the cylinders were firing.
These cars are far from complex and simple time tested methods can work quickly.
Pull off the injector connector for each cylinder in turn, the missing cylinder will have little effect on the engine tone. You may not notice much in the way of a rpm drop as the idle stepper motor will keep the idle speed stable.
But you will notice the difference in engine tone.
Having diagnosed cylinder number 1 as the problem, the next step is to find out why.
You need 3 elements to make an engine run;
fuel.
spark.
compression.

Swopping the injector connector with cyl 2 did not move the problem so a fault within the fuel system could be quickly ruled out.
That left the ign and engine condition.
A quick spark check showed a spark at the end of plug lead. But that isn't the end of the spark test. We need to make sure its not the plug. Simple (on these cars) whip out the plug and see if it sparks when earthed while the engine is running.

While the plug is out carry out a quick compression check, compare results with a running cylinder if the cranking pressure is unkown.
In this case the plug had failed.

Using an automotive scope and amps clamp would of proved the fault much quicker and prevented miss diagnosis.