The Sienna lit up the tire pressure warning light and the ABS trouble light on the trip from Rochester. The pressures were OK, if a bit low, but the early Toyota TPMS used wheel rotation sensors rather than direct pressure sensors, and we suspect a sensor went bad.
The ABS doesn’t report errors through the OBD II interface, requiring a jumper between TC and E1 in the ABS diagnostic interface block under the hood. Our Larval Engineer shows much respect for the engineer who included the pin ID layout under the flip-top lid, eliminating the need for scratch paper.
Despite diligent searching, there seems to be no Official Documentation of the blink codes appearing on the ABS trouble indicator. Fragmentary evidence suggests that a table applying to a Toyota MR2 MKII sports car would be generally applicable, which is hereby ripped to forestall link rot:
| Code Number | Diagnosis |
| 11 | open circuit in solenoid relay circuit |
| 12 | short circuit in solenoid relay circuit |
| 13 | open circuit in pump motor relay circuit |
| 14 | short circuit in pump motor relay circuit |
| 21 | open or short circuit in 3 position solenoid of front right wheel |
| 22 | open or short circuit in 3 position solenoid of front left wheel |
| 23 | open or short circuit in 3 position solenoid of rear wheels |
| 31 | front right wheel speed sensor signal malfunction |
| 32 | front left wheel speed sensor signal malfunction |
| 33 | rear right wheel speed sensor signal malfunction |
| 34 | rear left wheel speed sensor signal malfunction |
| 35 | open circuit in front left or rear right wheel speed sensor |
| 36 | open circuit in front right or rear left wheel speed sensor |
| 41 | abnormal battery voltage ( < 9.5 or > 17 ) |
| 51 | pump motor of actuator locked or open circuit in pump motor circuit in actuator |
| ALWAYS ON | computer malfunction |
The 3-4 blink code indicates a left rear wheel sensor failure. Such sensors (or their cables) seem to be either $35 or $175 from the usual sources, with no indication of why some are far more expensive than others. The pictures and descriptions are unhelpful, to say the least.
We’ll try cleaning the sensor, which probably won’t improve the situation, and then replace the poor thing.
The Smell of Molten Projects in the Morning 
How does a wheel rotation TPMSensor work, I wonder.
It’s brutally simple: long-term, a wheel that spins at a different speed than the others has the wrong pressure! I think high pressure = lower speed, low pressure = higher speed, but maybe they’re just looking at the absolute value of the difference.
However, the typical failure is for all four tires to slowly lose pressure (because nobody checks the tire pressure), so they all rotate at the same speed and the TPMS system remains blissfully ignorant. I think the standards now call for in-wheel pressure monitoring and wireless data transmission.
ABS uses the short-term relative rotation to detect wheel spinning: if a drive wheel spins faster, it’s slipping. If a braked wheel spins slower, it’s skidding.
” the typical failure is for all four tires to slowly lose pressure (because nobody checks the tire pressure), so they all rotate at the same speed and the TPMS system remains blissfully ignorant” Yep. That was why I was wondering how it would work. What decent engineer would let that out the door?
I think ABS came first, so you’re given four rotational sensors and can build a TPMS to detect a catastrophic flat or single-tire leak with no additional hardware: ain’t to be sniffed at.
The always-reliable Wikipedia sayeth the ABS mandate happened in 2011 and TPMS in 2007, but the 2000 Sienna has both. So I think they were just trading off optional capability with cost, which is what engineering is all about.
Want a fancier TPMS? Buy a fancier car! [grin]
Obligatory XKCD: https://xkcd.com/277/
I wasn’t thrilled to see that the Forester had 4 TPMS sensors allowed, thus requiring me to get snow/summer tires registered, or buy the tools to do it. Either that or get used to the {!} icon every winter. [grin]
Haven’t decided if TPMS is worth the hassle. At least, I check the tires every winter/summer… (Spare, too.)