Adventures of a DIY Miata Enthusiast: Code P0470

Hi Folks,

I've been spending a lot of time around my car (1997 Mazda Miata) trying to get it to pass emissions. She sat for about 3 years while I lived in DC and just last year, I decided to resurrect her back to life.

As many Marylander's know (and I'm assuming most states across the US), emissions is one of those things every car owner can count on, like death and taxes. When I got my car up and running (and the smoke cleared away), there was a check engine light illuminated on my dash. After using a code reader, I ended up with P0470 - Exhaust Pressure Sensor Circuit Malfunction. (huh?) Five month's later, I had a $1000 tab that probably didn't need to be spent and a car that worked like a charm. I took it to a mechanic and a dealer - neither of which solved the problem, but they sure threw a lot of parts at it (the dealer in particular was totally unhelpful - and I still feel like although I paid for an EGR Service Kit that I didn't get all the pieces included for the price). While this particular repair is somewhat specialized, I'll eventually post repair documentation for things like brakes, rotors, oil changes, and other regular maintenance items that a DIY'er could save a lot of money on by doing themselves. In general I go to the Miata.net forum as a first reference for any problems. There is a wealth of information there and an army of enthusiasts that are more than willing to provide great diagnostic advice for some of those tough problems. In particular 92MarinerBlue was a huge help in working through the P0470 problem I had, detailed in this post.

Here is a web link to the photo-documentation of a complete removal, diagnostic and repair (where necessary) of the car's EGR system. It was a mess that took about 5 months to clean up, but had a happy ending!

The following repairs are covered:

- Service the EGR Boost Sensor (which relocated it to the rear wall behind the engine)
- Removal and cleaning of the EGR Valve
- Removal and cleaning of the EGR Pipe
- Removal of the Throttle body and cleaning of the EGR tunnel that goes from the front of the intake manifold (just behind the throttle), all the way to the back where it connects with the top opening of the EGR valve.

My P0470 was caused by a physical clog in the intake manifold where the EGR tunnel meets the larger chambers of the intake. Upon fixing that, I created a few new problems:
- EGR Valve was always open at idle: this was caused by accidentally swapping the vacuum hoses from the solenoids creating a constant one-way vacuum. Fixed by swapping the hoses. (Doh!)
- Fluctuations in Idle speed: This was caused be replacing the throttle gasket. It was fixed by lowering the idle which I had gradually increased over the years due to performance issues (now I understand that it was actually because of a clog in the intake manifold).

If anyone is experiencing problems with their EGR sensors and the P0470 code, take a look at this post before you go and buy a bunch of new sensors. I'm sure that my original boost sensor and CV purge valves were just fine and it was a waste of several hundred dollars.

Click on the picture below to get started...