The EGR System tends to get restricted or blocked with carbon after 100,000- 125,000 miles which will cause an emissions test failure for NOX. If the EGR system is equipped with an EGR temperature sensor it will trigger a Check Engine Light for improper EGR flow. The repair is to clean out the EGR passages and the EGR Temperature sensor. Our technicians tell this repair is pretty straight forward and takes about 1-1.5 hours. It is also wise to verify the EGR system components i.e. the Transducer, EGR Valve and VSV Solenoid at this time.Google+
Car Problem Reports
Toyota Previa Failed Emissions Test Due to Blocked EGR System
Toyota Previa Problem
Average Mileage: 195,875 mi (170,000 mi - 256,000 mi)
Visitor, 1992 Toyota Previa, 170,000 mi
Removed EGR valve and egr vacuum. Need a 24mm open-end wrench to loosen the EGR valve nut in back, which stays with the engine side so loosen it cw from your point of view.
Mark the egr vacuum and the hoses so you get the two back on correctly.
Carburetor cleaner soaked the EGR valve and scrubbed it out with pipe cleaners. Removed gas temp sensor real quick with 14mm open-end. Sprayed it clean too. With the EGR out of the way you can see this bolt and wire clearly. To get this out you must first disconnect the plastic electrical joint so the wire spins freely. Typical reports say that the wiring here is often the problem. Maybe. So connect 'em real good.
I have a multimeter tester now to check these things when an error light comes on. Throttle position sensor is my next project for error code 41.
Error code 71 cleared.
Visitor, 1992 Toyota Previa, 182,000 mi
Check engine light is on with a code 71. I plan on cleaning the EGR and Oxygen sensor.
High NOX. Passed test but getting close.
Visitor, 1994 Toyota Previa, 180,000 mi
So I had this problem and I started with the fuel temp sensor, had a bad OHM reading, replaced that, changed plug wires and plugs, put in an after market cat converter, Problematic but solved to an easy installation by cutting of the front flange of the old converter , flared the new converter's end pipe by slowly but surely beating it from the inside out over the edge of my vice using a small ball pien hammer. When it was flared sufficiently for a good diameter match to the old flange, I simply brazed the two together paying attention to positioning of the flange to the converter. The rear converter pipe had to be cut a bit to shorten the assembly. I found an adapter at Advanced auto to make the transition from the Converter to the old exhaust pipe but pushed it together over the rear nipple of the new converter with a good smear of candle wax on both pieces,and a little persuasion with a dead blow hammer. Clamped that all together at the rear and put new bolts thru the front flange with a new gasket. The new converter assembly factory part is over $800, and my solution cost my time and an $85. All that reduced my NOx from 8000 to 4000.444, still failing emmisions so now here we go with the EGR. I have the shop manual on the van so that helps a lot. I let you know about my EGR experience.
Visitor, 1992 Toyota Previa, 188,000 mi
Following up on report #2 in which i described clearing error code 71:
This time i was going after error code 41... In my '92 Previa, the old oxygen sensors are still working fine. What failed on the smog test was NOx. That needed a timing adjustment and an EGR pipe cleaning. AND the error code 41 was still on.
For the error you can pursue the Throttle position sensor, but to get it off you have to remove the throttle body, including the rubber air intake which may be brittle up around the throttle and possibly crack off. If you do all that you can test the TPS with a multimeter for ohms to match what's in the repair manual.
BUT, first i'd suggest you take an easier route... Unbolt the Computer, under the driver seat. Not the little grey plastic box but the big flat metal box next to it. 4 wires run from the tps to the computer box. You can see the colors yellow-red, blue-black, brown, turquoise-black. probably. Using a new wire temporarily replace the blue-black. If that isn't the short try one by one the other 3 wires or more than one at a time. Pull out the female connectors to see what you're doing.
RE-string the wires if you have time. They are fairly short and easy to get at once you have the ECU box in hand.
To clear an error 41 after it's been fixed, you may even need to use 2 probes to ground TC and AB as in resetting the airbag light. see this link for how-to:http://wiki.answers.com/Q/How_do_you_reset_airbag_light_on_a_1995_toyota_tocoma
Otherwise, if check engine light is out, you're good to go.
Visitor, 1991 Toyota Previa, 256,000 mi
Engine check light appeared. The mechanic determined that it was code 71 and he fixed the egr system by replacing egr sensor and egr valve.