I’m going to share how I fixed a P0121 on a VW Tiguan 2.0 TDi by cleaning the throttle valve.
The engine light wasn’t on, but I found the code whilst fixing the boot latch. As you’ll see from what I found, it was only a matter of time before this would trigger the engine light.
The vehicle is a 2012 VW Tiguan 2.0 TDi DSG 4Motion with the CFFB engine and about 83,000 miles. It doesn’t do many long journeys.
I’ve worked on a few VWs and Audis over the years, so use VCDS as my diagnostic tool for VAG cars.
- T30 Torx bit and driver.
- 10mm spanner.
- Lots of clean rags or paper towels.
- Solvent for cleaning.
- Blunt screwdriver or other similar tool for scraping debris.
- Code reader.
Removing the throttle valve
As with much engine bay work, the first step is to remove a load of plastic gubbins out of the way. The engine cover is removed by pulling upwards.
I removed 2 parts of the air cleaner ducting to improve access: these being the cold air inake and the U shaped pipe linking this to the airbox. These just pull apart, although this can be made easier by pushing in the lock tabs holding the pieces together.
The next step was to undo 3 x T30 Torx bolts holding the throttle valve in place and detach the dipstick support bracket (the dipstick and tube can stay in place).
Unplug the connector, using a small screwdriver or plastic pry tool to lift the connector tab. I disconnected the battery negative terminal beforehand.
I put a clean glove over the end of the intake pipework, to prevent anything being dropped in there.
As soon as I removed the throttle valve, I felt there was a good chance I’d found the cause of the P0121 on the VW Tiguan: loads of carbon deposits inside the valve! The hypothesis being that the carbon on and around the throttle flap stops it from moving freely. This means the ECU gets an implausible signal from the throttle position sensor from time to time, since the flap isn’t in the position ‘expected’ by the ECU and reported by the sensor.
Obviously there is no certainty at this stage that cleaning the valve will solve the problem. The carbon deposits were sufficient to stop the throttle butterfly moving through its full range, so I felt it was worthwhile going ahead with the cleaning before embarking on any further diagnostic work. It’s possible you could do all this cleaning and not solve the problem, although I’d expect the engine light to be on in the case of an electronic failure in the valve motor or throttle position sensor.
Cleaning the throttle valve
This is a horrible job! I did it outside with gloves on, and worked on some cardboard that I could throw away afterwards. I also wore a mask to minimise inhalation of fumes from the solvent used for cleaning.
To start with I scraped the worst of the clag off with a blunt screwdriver (blunt so as not to score the throttle valve housing or plate).
Next I used Wynn’s diesel EGR extreme cleaner (part number 23379). Like many products of its type, most of the instructions are about spraying it into the intake with the engine running to clean the EGR valve. However it is also designed for direct application to carbon deposits on intake components.
It took a while, but I’m not surprised given the level of carbon deposits on the valve. Eventually I was left with a lovely clean throttle valve. I was glad I’d got 2 cans!
Refitting the throttle valve
This is a straightforward reversal of the removal process. I used a new gasket from Elring (part number 006.070).
Deleting the P0121 from the VW Tiguan
Having put everything back together, it was time to go back to VCDS and delete the P0121 code from the Tiguan. By the way, you don’t need VCDS to do this job. It’s a great piece of kit for detailed diagnostics and repairs, but for this any standard OBD2 code reader would have been sufficient.
I deleted the code and started the engine. Everything was fine and the code didn’t come back. Always a good feeling!
I noticed the improved throttle response immediately on the test drive, and the acceleration felt a lot more brisk than before the repair. Making a real improvement in the driving experience was even more rewarding than getting rid of the code.
I did another code scan to check the code (or any others) hadn’t appeared after the test drive. It hadn’t, so good news!
The Tiguan owner was really impressed with the improvement, and was particularly pleased to see improved fuel economy after the throttle valve cleaning.
As a final note, there is an argument that the EGR valve should be cleaned too, because if the throttle valve is like this one was, it’s likely the EGR valve is too. I didn’t clean the EGR valve this time, but acknowledge it would be ‘best practice’ to do so.
I’ve also advised the owner to sell the car! That might seem a bit excessive, but this engine isn’t (and diesel engines in general aren’t) best suited to lots of short journeys with no long journeys.
Here is a video of me working on the car: