P0420 the most common fault code?

P0420 is possibly the most common DTC retrieved from modern vehicles.
This is because it is one of the codes that is concerned with the exhaust system, any fault upstream of the exhaust can result in a P0420 code.

This why it can be cleared and not return or cleared only to return but not always immediately.
The catalyst can be damaged by many things, but misfire is the most common cause of catalyst damage. P0420 indicates the catalyst is operating below some threshold set within the ECU software. It can be a temporary fault or an indication of catalyst failure.

It is monitored by the O2 sensors, one before the catalyst and one after the catalyst. The two signals are compared and if they are similar the ECU flags the code.

Above is a scan tool image of a catalyst operating efficiently.



There are number of ways to test the catalyst efficiency

• use an infra red thermometer to check for an increase in temperature across the catalyst
• Scan tool data to monitor the switch ratio before and after the catalyst
• oscilloscope to check O2 signal output
• Gas analyser to check readings
• EOBD mode 6

Always test the catalyst under the most favourable conditions, as this is how the ECU tests the catalyst. Hot engine, fast idle/cruise and no other faults present.

The ECU will only monitor the the catalyst efficiency if there are no codes stored. Therefore it is essential to check the catalyst before returning the vehicle to the customer, if a fault has been repaired that may have damaged the catalyst, such as coil pack failure.

Remember it is the O2 sensor that is used to monitor the catalyst so these must be operating correctly.





P0420 Jaguar which bank is bank 1

This a common query with "V" engines the ECU has flagged a code that relates to Bank 1.
However which bank is bank 1?
That's easy, it is the bank that contains cylinder number 1.
So in this case which bank has cylinder number 1 is it the right hand or left hand bank.
Without any technical data, this can become a bit of a puzzle but an easy way to find out is remove the coil pack or injector plug from one of the cylinders, this will flag a code relating to the offending cylinder. That way you can identify cylinder 1 and therefore bank 1. In this case the right hand bank looking from the drivers seat.

But what about the code, what is it trying to tell you?
More on that next time.

P0251 Ford Focus under pressure

This 2003 Ford Focus 1.8TDCi had refused to start one morning, the owner had coaxed it into life with a sniff of easy start.
It had been playing up for a while, under load the glow plug light would flash and the car would loose all performance, now it refuses to start without the help of easy start.

A quick visual check and it looked like the previous garage had thrown some parts at it in an attempt to diagnose the cause. It sported a new fuel filter, new inlet metering valve and a couple of cans of injector cleaner. None of these had cured the fault and the final diagnosis was the high pressure pump.

What had made the garage come to this conclusion?


This code was stored, but after a test drive more codes

These suggest that the rail pressure cannot be controlled within the limits programmed into the ECU. A new fuel metering valve has been fitted so it must be the pump at fault.

Or is it? Rail pressure deviation has a number of causes, and we split them into supply pressure faults, high pressure faults, fuel delivery and return faults.

Using a graphing scan tool it is possible to test the high pressure generation using serial data. The system needs around 200-250 bar before it will switch the injectors.

We had 84 bar cranking, the fuel metering valve can be disconnected on this system and the test repeated and should result in full system pressure (no control) this resulted in 194 bar. We had a high pressure generation fault. But it can not be the metering valve causing the problem.

The scan tool can provide data about fuel delivery and the PID to check is the correction factor for the individual cylinders. Common rail injectors normally have a high return flow rate when they are in trouble so with the excessive correction factor in cylinder 1 we expected the worst when carrying out the back leak test. So it proved, cylinder 1 had twice the return flow of the other injectors.

A new injector was fitted and coded into the ECU. The injector is about 1/10th the price of the pump. But did it fix the pressure fault?

Cranking with the cam sensor disconnected prevents the ECU switching the injectors, and allows the pressure to build in the rail. We now had just over 600 bar cranking.This test proves the pump and low pressure stage as any faults would not generate such high pressures.

Losing Focus

This looked like a simple diagnosis.

A 2005 Ford focus1.6TDCI with the MIL light on, and black smoke coming from the exhaust during hard acceleration.

First we performed a visual inspection and noticed a new Air mass meter(AMM). The test drive proved the customer was right, black smoke and poor vehicle performance.

Time to attach the code reader.

P1101 was the code stored. This relates to the air mass meter out of self test range.
possible causes for this code include;
Low Battery Voltage
AMM partially connected
AMM contaminated
Ground or power supply problems
AMM open circuit to ECU
AMM damaged
ECU damage

But as the component is new, and the customer will be less than pleased if the diagnosis is the same as a few months ago, and the light comes back on with the same symptoms. The pressure is now on. What is causing the AMM to under read, a classic cause and effect diagnostic problem. The previous garage has fixed the effect, and the problem has returned, we need to establish the cause.

This is where information can help, the diagnostic triangle consists of three elements, equipment, information and knowledge. All three are required to diagnose vehicles effectively.



We discovered that the problem can be caused by the crankcase breather pipe contaminating the AMM, and ford has modified the pipe to help prevent the problem returning. They also reprogram the ECU with larger performance parameters to prevent the MIL illuminating.


I checked the software number and the vehicle needed the software upgrades, along with the modified pipe. However the AMM was now contaminated, as it was still relatively new I cleaned it with brake cleaner and hoped for the best. On test drive the vehicle performed as it should, and there was no evidence of the black smoke. Satisfied we had found the cause, the car was returned to the customer.

Out of Ranger

A Ford Ranger had turned up at a garage overnight, with the keys posted through the letter box and a note asking if the garage could get it going.

The problem here is not fixing the vehicle but a lack of information.

1, how did the fault occur
2, when did it happen
3, with which key(s)
4, how much are you willing to spend

The customer was contacted and answered all the questions, but fustrated at the lack of diagnosis, surely you can just plug it in and find out what's wrong?

The garage did not have the diagnostic plug to tackle the job and asked if I could have a look. I found a code stored P1649 pump control module malfunction.

After a quick check of the supply and grounds this looked like a ECU fault.
The module was removed and sent away for repair, it was repaired but the vehicle still would not start. Next step was to check the engine ECU, a separate ECU that shares inputs and outputs with the pump control module. This too appeared to be in trouble. So this was sent away for testing and repairs.

What had caused the damage to both control units and is the problem going to destroy the repaired units?

Further testing didn't show any problems. The customer told us upon collection he had tried jump starting the vehicle when it refused to start, and thinks he may have got the leads the wrong way round!

VW EPC Light

I often get asked why the EPC light can be illuminated without the MIL light.

EPC -electronic power control consists of the pedal position sensors, MAF sensor, ECU, and throttle valve.
A classic input - logic -output system.
The ECU (ME7) has a torque based architecture, that is to say it manages outputs based on torque demands of the driver and the vehicle systems.

One of these outputs is the throttle valve.

If a fault occurs in the throttle valve, ECU or wiring, the ECU defaults into a limp home mode.
The Electronic Power Control (EPC) light will illuminate.

Depending on the fault one of three modes can be selected.

Mode 1
If one throttle position sensor fails, torque output is reduced.

The customer may complain of a lack of power.
This mode requires one functional throttle position sensor with an intact signal, and
engine load readings from the Mass Air Flow (MAF) sensor.

Mode 2
If the throttle valve actuator fails or malfunctions, the voltage to it is shut off and the
throttle valve defaults to its emergency running position. Torque requests are executed via ignition and charge pressure regulation. As a result, the engine will show very little
response to the throttle. This mode requires signals from both throttle position sensors.

Mode 3
If the throttle valve position sensor fails (implausible signals), the voltage to it is shut off and the throttle valve goes to its emergency running position. The engine speed is limited to 1200
RPM by restricting the fuel injector pulse width and ignition timing.

Therefore a technician can predict the fault from observing which limp home mode has been selected. As the engine continues to run correctly, there is no excessive emissions and therefore no MIL.

Vaux Pop P1120 P1550

This 2001 Zafira (engine code Z18XE) came from another garage, who had extracted the fault codes. The codes were P1120 (Accelerator Pedal Position Sensor 1 Low Input)
and P1550 (Electronic Throttle Control Reduced Power)
They looked at the live data and pedal position sensor 1 did not respond to the pedal movement.
So it looked like a simple sensor fault, but another garage had replaced the pedal box containing the sensors and the fault remained. They had also removed and cleaned the throttle, and could not perform the throttle re-learn using their diagnostic tool.
Due to the cost of the repairs so far, they asked for a second opinion.
As always we started with a visual inspection. Then confirmed the fault, poor performance, stalling and MIL on. Then we looked for fault codes in all modules as the TCS light was also flashing.
The same codes were presented in the engine control module.
This is where system knowledge is a key ingredient to your diagnostic routine.
P1550 is stored as a result of the P1120 code, it is a symptom not a cause. The ECU has gone into a safe mode of operation. (reduced power mode).
So what could be the cause of the pedal position sensor 1 low input?
The list includes, wiring faults, sensor faults, Module faults. How to work out which one is the cause requires some information. Using a wiring diagram we are able to determine which ECU pins were supplies, earths and signals. A multi-meter could then be used to test the wiring, we elected to test at the ECU. This allows you to test the entire circuit at once.
The supplies and earths were good, and both sensors appeared to be working. One sensor output is twice the other.
This means the fault is inside the ECU. Not uncommon with this engine.
Now the customer is left with a choice, new ECU and coding at the Vauxhall dealers, or a refurbished second unit. The customer sourced his own ECU and it came with the transponder and key chips. Once fitted it required no coding, other than a throttle reset which was carried out using the original repairers equipment.

Feeling Cranky?

We were called to look at a race car that had a Ford Cosworth V6 engine fitted in place of the original Essex V6. This conversion should in theory free up 50-60 bhp, however the engine would not fire.

This engine ran the old Ford EDIS ignition set up, with a separate ecu providing the fuel.

The fuel ecu modifies the ignition timing after receiving a digital signal from the EDIS module. It uses this signal for its speed/position reference.

As there was no fuel or spark it seemed logical to start with this signal. But only after checking for the correct lives and earths at both ecus. We corrected a number of wiring errors, and thought we might have sorted the problem. But it still refused to fire up.

We checked for the PIP signal.

PIP is the Ford term for the speed/position reference signal (profile ignition pickup).

There was no output from the EDIS. Next step is to check the input.

This is a simple inductive crank sensor from the front pulley. There was a signal, however it was different from the expected signal. We suspected the crank sensor was the culprit. A quick resistance check showed a normal 0.8K ohms, this is why an oscilloscope is essential, if we had just used a meter, we would have an AC frequency and the correct resistance readings. This would then suggest a fault with the EDIS module.

Input - logic- Output model would lead to miss diagnosis of the module as there was no output despite the correct power, ground and inputs.

Unable to find a replacement sensor on the self we rigged up a CKS from a Renault using a cable tie and two jumper wires. result is shown below.

The yellow trace is the CKS and the green trace the PIP.

The ECU was now injecting and the coil pack sparking. The difference between the two CKS traces is the waveform around the missing tooth. The jump in the waveform is due to the spark plugs being out of the engine causing RFi made worse by the exposed makeshift CKS wiring.

Another successful diagnosis due to the oscilloscope being able to display the full picture.

Fault layers

Vehicle diagnosis is like an onion it can have many layers and make your eyes water.

I was called to a garage to check out a Toyota Rav 4 they had in for diagnosis. The vehicle was very flat hardly able to pull away under its own steam. The fault history had been cleared and the only fault remaining was O2 sensor bank 2 sensor 1. Unsure which sensor is bank 2 sensor 1 they were going to order a pair. The price soon put a stop to that plan. This vehicle is fitted with wideband sensors which are much more expensive than the narrow band type. I did a quick inspection of the wiring, and quickly tested the heater circuits using a scope. Bank 2 sensor 1 had a open circuit heater. A new sensor was ordered, how ever this was unlikely to cure the flat running problem.
I had a quick look at the live data, and noticed the air mass was under reading at WOT. I checked this with a scope and sure enough it was only hitting 2.7V on snap acceleration. It should be closer to 4.5V. I checked the supply and earth connections both were fine. So a new air mass meter was added to the order.
After both components had been fitted and the codes cleared the vehicle displayed normal driving characteristics. As is often the case the fault code and mil light did not relate to the symptoms. The customer wanted the light switched off, so I guess that renewing the O2 sensor would of fixed that, but the air mass was causing the flat running. This is typical of engine management faults when you will be presented with multiple codes, or multiple faults or even both. You have to peel these layers back to reveal the fault(s). The cost of the parts can make your eyes water.